diff --git a/model_cluster.go b/model_cluster.go
index e431cdc..a5ceaa2 100644
--- a/model_cluster.go
+++ b/model_cluster.go
@@ -226,13 +226,13 @@ func (l *clusterImpl) estimateDeltaScore(
 			return deltaScore, constNoPositionsHint
 		}
 
-		candidate := stopPosition.stop()
+		candidate := stopPosition.Stop()
 
 		var c *centroidData
 		if asConstraint {
-			c = vehicle.last().ConstraintData(l).(*centroidData)
+			c = vehicle.Last().ConstraintData(l).(*centroidData)
 		} else {
-			c = vehicle.last().ObjectiveData(l).(*centroidData)
+			c = vehicle.Last().ObjectiveData(l).(*centroidData)
 		}
 
 		centroid := c.location
@@ -249,7 +249,7 @@ func (l *clusterImpl) estimateDeltaScore(
 					continue
 				}
 				centroidOtherVehicle := otherVehicle.
-					last().
+					Last().
 					ConstraintData(l).(*centroidData).location
 
 				if haversineDistance(
@@ -285,12 +285,11 @@ func (l *clusterImpl) getSolutionStops(vehicle SolutionVehicle) []SolutionStop {
 }
 func (l *clusterImpl) Value(solutionStop Solution) float64 {
 	sum := 0.0
-	for _, v := range solutionStop.(*solutionImpl).vehiclesMutable() {
-		vehicle := v.(solutionVehicleImpl)
+	for _, vehicle := range solutionStop.(*solutionImpl).vehiclesMutable() {
 		if vehicle.IsEmpty() {
 			continue
 		}
-		sum += vehicle.last().ObjectiveData(l).(*centroidData).compactness
+		sum += vehicle.Last().ObjectiveData(l).(*centroidData).compactness
 	}
 	return sum
 }
diff --git a/model_constraint_maximum_duration.go b/model_constraint_maximum_duration.go
index bff13cc..5d5fb3e 100644
--- a/model_constraint_maximum_duration.go
+++ b/model_constraint_maximum_duration.go
@@ -60,7 +60,7 @@ func (l *maximumDurationConstraintImpl) EstimateIsViolated(
 
 	maximumValue := l.maximum.Value(vehicleType, nil, nil)
 
-	startValue := vehicle.first().StartValue()
+	startValue := vehicle.First().StartValue()
 	previous, _ := moveImpl.previous()
 	endValue := previous.EndValue()
 
diff --git a/model_constraint_maximum_stops.go b/model_constraint_maximum_stops.go
index 6839058..17b3091 100644
--- a/model_constraint_maximum_stops.go
+++ b/model_constraint_maximum_stops.go
@@ -57,7 +57,7 @@ func (l *maximumStopsConstraintImpl) EstimateIsViolated(
 	stopPositions := moveImpl.stopPositions
 	nrStopsToBeAddedToSolution := len(stopPositions)
 
-	beforeStop := stopPositions[len(stopPositions)-1].next()
+	beforeStop := stopPositions[len(stopPositions)-1].Next()
 	vehicle := beforeStop.vehicle()
 
 	vehicleType := vehicle.ModelVehicle().VehicleType().Index()
diff --git a/model_constraint_maximum_travel_duration.go b/model_constraint_maximum_travel_duration.go
index 2d28326..1e36f5a 100644
--- a/model_constraint_maximum_travel_duration.go
+++ b/model_constraint_maximum_travel_duration.go
@@ -94,9 +94,8 @@ func (l *maximumTravelDurationConstraintImpl) EstimateIsViolated(
 }
 
 func (l *maximumTravelDurationConstraintImpl) DoesVehicleHaveViolations(vehicle SolutionVehicle) bool {
-	vehicleImpl := vehicle.(solutionVehicleImpl)
-	return vehicleImpl.last().CumulativeTravelDurationValue() >
-		l.maximum.Value(vehicleImpl.ModelVehicle().VehicleType(), nil, nil)
+	return vehicle.Last().CumulativeTravelDurationValue() >
+		l.maximum.Value(vehicle.ModelVehicle().VehicleType(), nil, nil)
 }
 
 func (l *maximumTravelDurationConstraintImpl) IsTemporal() bool {
diff --git a/model_constraint_no_mix.go b/model_constraint_no_mix.go
index bcf6b1f..a508cb8 100644
--- a/model_constraint_no_mix.go
+++ b/model_constraint_no_mix.go
@@ -349,19 +349,19 @@ func (l *noMixConstraintImpl) EstimateIsViolated(
 	move SolutionMoveStops,
 ) (isViolated bool, stopPositionsHint StopPositionsHint) {
 	moveImpl := move.(*solutionMoveStopsImpl)
-	_, hasRemoveMixItem := l.remove[moveImpl.stopPositions[0].stop().ModelStop()]
+	_, hasRemoveMixItem := l.remove[moveImpl.stopPositions[0].Stop().ModelStop()]
 	if hasRemoveMixItem {
 		return true, constNoPositionsHint
 	}
 
-	previousStopImp := moveImpl.stopPositions[0].previous()
+	previousStopImp := moveImpl.stopPositions[0].Previous()
 	previousNoMixData := previousStopImp.ConstraintData(l).(*noMixSolutionStopData)
 	contentName := previousNoMixData.content.Name
 	contentQuantity := previousNoMixData.content.Quantity
 
 	deltaQuantity := 0
 
-	insertMixItem, hasInsertMixItem := l.insert[moveImpl.stopPositions[0].stop().ModelStop()]
+	insertMixItem, hasInsertMixItem := l.insert[moveImpl.stopPositions[0].Stop().ModelStop()]
 	if hasInsertMixItem {
 		if contentName != insertMixItem.Name && previousNoMixData.content.Quantity != 0 {
 			return true, constNoPositionsHint
@@ -377,14 +377,14 @@ func (l *noMixConstraintImpl) EstimateIsViolated(
 	}
 
 	for idx := 1; idx < len(moveImpl.stopPositions); idx++ {
-		previousStopImp = moveImpl.stopPositions[idx].previous()
+		previousStopImp = moveImpl.stopPositions[idx].Previous()
 		if previousStopImp.IsPlanned() {
 			previousNoMixData = previousStopImp.ConstraintData(l).(*noMixSolutionStopData)
 			if previousNoMixData.tour != tour || previousNoMixData.content.Name != contentName {
 				return true, constNoPositionsHint
 			}
 		}
-		insertMixItem, hasInsertMixItem = l.insert[moveImpl.stopPositions[idx].stop().ModelStop()]
+		insertMixItem, hasInsertMixItem = l.insert[moveImpl.stopPositions[idx].Stop().ModelStop()]
 		if hasInsertMixItem {
 			if contentName != insertMixItem.Name {
 				return true, constNoPositionsHint
@@ -392,7 +392,7 @@ func (l *noMixConstraintImpl) EstimateIsViolated(
 			deltaQuantity += insertMixItem.Quantity
 			continue
 		}
-		removeMixItem, hasRemoveMixItem := l.remove[moveImpl.stopPositions[idx].stop().ModelStop()]
+		removeMixItem, hasRemoveMixItem := l.remove[moveImpl.stopPositions[idx].Stop().ModelStop()]
 		if hasRemoveMixItem {
 			if contentName != removeMixItem.Name || contentQuantity+deltaQuantity < removeMixItem.Quantity {
 				return true, constNoPositionsHint
diff --git a/model_latest.go b/model_latest.go
index d52ce9b..ffd8bbc 100644
--- a/model_latest.go
+++ b/model_latest.go
@@ -190,8 +190,8 @@ func (l *latestImpl) Value(s Solution) float64 {
 	solution := s.(*solutionImpl)
 	value := 0.0
 	for _, vehicle := range solution.vehicles {
-		solutionStop := vehicle.first().Next()
-		lastSolutionStop := vehicle.last()
+		solutionStop := vehicle.First().Next()
+		lastSolutionStop := vehicle.Last()
 		for {
 			latenessFactor := l.latenessFactor.Value(
 				nil,
@@ -241,7 +241,7 @@ func (l *latestImpl) estimateDeltaScore(
 	first := true
 
 	arrival, start, end := 0.0, 0.0, 0.0
-	previousStop := vehicle.first().ModelStop()
+	previousStop := vehicle.First().ModelStop()
 	generator := newSolutionStopGenerator(*move, false, true)
 	defer generator.release()
 
diff --git a/model_maximum.go b/model_maximum.go
index f08b3c5..a5d9268 100644
--- a/model_maximum.go
+++ b/model_maximum.go
@@ -208,7 +208,7 @@ func (l *maximumImpl) EstimateIsViolated(
 	// level at the end of the vehicle. We can only do this if the expression
 	// is a stop expression.
 	if l.hasStopExpressionAndNoNegativeValues {
-		cumulativeValue := vehicle.last().CumulativeValue(expression)
+		cumulativeValue := vehicle.Last().CumulativeValue(expression)
 
 		if cumulativeValue+l.deltas[moveImpl.planUnit.modelPlanStopsUnit.Index()] > maximum {
 			return true, constSkipVehiclePositionsHint
@@ -241,7 +241,7 @@ func (l *maximumImpl) EstimateIsViolated(
 
 	if !l.hasNegativeValues {
 		violated := level-previousStop.CumulativeValue(l.Expression())+
-			vehicle.last().CumulativeValue(l.Expression()) > maximum
+			vehicle.Last().CumulativeValue(l.Expression()) > maximum
 		return violated, constNoPositionsHint
 	}
 
@@ -304,7 +304,7 @@ func (l *maximumImpl) EstimateDeltaValue(
 
 	vehicle := moveImpl.vehicle()
 
-	hasViolation := vehicle.last().ObjectiveData(l).(*maximumObjectiveDate).hasViolation
+	hasViolation := vehicle.Last().ObjectiveData(l).(*maximumObjectiveDate).hasViolation
 
 	vehicleType := vehicle.ModelVehicle().VehicleType()
 	maximum := l.maximumByVehicleType[vehicleType.Index()]
@@ -325,7 +325,7 @@ func (l *maximumImpl) EstimateDeltaValue(
 	// level at the end of the vehicle. We can only do this if the expression
 	// is a stop expression.
 	if l.hasStopExpressionAndNoNegativeValues {
-		cumulativeValue := vehicle.last().CumulativeValue(l.resourceExpression)
+		cumulativeValue := vehicle.Last().CumulativeValue(l.resourceExpression)
 
 		returnValue := 0.0
 		excess := cumulativeValue + l.deltas[moveImpl.planUnit.modelPlanStopsUnit.Index()] - maximum
@@ -394,7 +394,7 @@ func (l *maximumImpl) Value(
 		maximum := l.maximumByVehicleType[vehicleType.Index()]
 
 		if l.hasStopExpressionAndNoNegativeValues {
-			cumulativeValue := vehicle.last().CumulativeValue(l.resourceExpression)
+			cumulativeValue := vehicle.Last().CumulativeValue(l.resourceExpression)
 			excess := cumulativeValue - maximum
 			if excess > 0 {
 				score += excess
diff --git a/model_objective_earliness.go b/model_objective_earliness.go
index 759d49c..e7291f3 100644
--- a/model_objective_earliness.go
+++ b/model_objective_earliness.go
@@ -123,7 +123,7 @@ func (l *earlinessObjectiveImpl) EstimateDeltaValue(
 	// Init data
 	first := true
 	arrival, start, end := 0.0, 0.0, 0.0
-	previousStop := vehicle.first()
+	previousStop := vehicle.First()
 
 	// Get sequence starting with the first stop prior to the first stop to be
 	// inserted.
diff --git a/model_objective_travelduration.go b/model_objective_travelduration.go
index dfce85b..0b4dbea 100644
--- a/model_objective_travelduration.go
+++ b/model_objective_travelduration.go
@@ -28,7 +28,7 @@ func (t *travelDurationObjectiveImpl) Value(solution Solution) float64 {
 
 	score := 0.0
 	for _, vehicle := range solutionImp.vehicles {
-		score += vehicle.last().CumulativeTravelDurationValue()
+		score += vehicle.Last().CumulativeTravelDurationValue()
 	}
 	return score
 }
diff --git a/model_objective_vehicles_duration.go b/model_objective_vehicles_duration.go
index b66ff58..023e883 100644
--- a/model_objective_vehicles_duration.go
+++ b/model_objective_vehicles_duration.go
@@ -58,7 +58,7 @@ func (t *vehiclesDurationObjectiveImpl) EstimateDeltaValue(
 
 	first := true
 	end := 0.0
-	previousStop := vehicle.first()
+	previousStop := vehicle.First()
 
 	generator := newSolutionStopGenerator(*solutionMoveStops, false, isDependentOnTime)
 	defer generator.release()
@@ -83,7 +83,7 @@ func (t *vehiclesDurationObjectiveImpl) EstimateDeltaValue(
 	nextmove, _ := solutionMoveStops.next()
 
 	if nextmove.IsLast() || isDependentOnTime {
-		return end - vehicle.last().EndValue()
+		return end - vehicle.Last().EndValue()
 	}
 
 	for solutionStop := nextmove.Next(); !solutionStop.IsLast(); solutionStop = solutionStop.Next() {
@@ -99,7 +99,7 @@ func (t *vehiclesDurationObjectiveImpl) EstimateDeltaValue(
 		}
 	}
 
-	last := vehicle.last()
+	last := vehicle.Last()
 	_, _, _, end = vehicleType.TemporalValues(
 		end,
 		last.Previous().ModelStop(),
diff --git a/solution.go b/solution.go
index 1c8c9ca..2e67eb0 100644
--- a/solution.go
+++ b/solution.go
@@ -129,7 +129,7 @@ func NewSolution(
 		model: m,
 
 		vehicleIndices:           make([]int, 0, nrVehicles),
-		vehicles:                 make([]solutionVehicleImpl, 0, nrVehicles),
+		vehicles:                 make([]SolutionVehicle, 0, nrVehicles),
 		solutionVehicles:         make([]SolutionVehicle, 0, nrVehicles),
 		first:                    make([]int, 0, nrVehicles),
 		last:                     make([]int, 0, nrVehicles),
@@ -395,7 +395,7 @@ func (s *solutionImpl) addInitialSolution(m Model) error {
 	PlanUnitLoop:
 		for _, planUnit := range planUnits {
 			stopPositions := make(StopPositions, 0, len(planUnit.SolutionStops()))
-			previousStop := solutionVehicle.first()
+			previousStop := solutionVehicle.First()
 
 			solutionPlanUnit := s.unwrapRootPlanUnit(planUnit)
 			allPlanUnits[solutionPlanUnit] = true
@@ -455,7 +455,7 @@ func (s *solutionImpl) addInitialSolution(m Model) error {
 						newStopPosition(
 							previousStop,
 							solutionStop,
-							solutionVehicle.last(),
+							solutionVehicle.Last(),
 						),
 					)
 				}
@@ -510,7 +510,7 @@ func (s *solutionImpl) addInitialSolution(m Model) error {
 					)
 				}
 				for _, position := range move.(*solutionMoveStopsImpl).stopPositions {
-					position.stop().detach()
+					position.Stop().detach()
 				}
 				infeasiblePlanUnits[solutionPlanUnit] = true
 				continue
@@ -594,7 +594,7 @@ type solutionImpl struct {
 	vehicleIndices       []int
 
 	// TODO: explore if vehicles should rather be interfaces, then we can avoid creating new vehicles on the fly
-	vehicles                 []solutionVehicleImpl
+	vehicles                 []SolutionVehicle
 	solutionVehicles         []SolutionVehicle
 	start                    []float64
 	slack                    []float64
@@ -664,17 +664,17 @@ func (s *solutionImpl) SolutionVehicle(vehicle ModelVehicle) SolutionVehicle {
 	if solutionVehicle, ok := s.solutionVehicle(vehicle); ok {
 		return solutionVehicle
 	}
-	return nil
+	return SolutionVehicle{}
 }
 
-func (s *solutionImpl) solutionVehicle(vehicle ModelVehicle) (solutionVehicleImpl, bool) {
+func (s *solutionImpl) solutionVehicle(vehicle ModelVehicle) (SolutionVehicle, bool) {
 	if vehicle != nil {
-		return solutionVehicleImpl{
+		return SolutionVehicle{
 			index:    vehicle.Index(),
 			solution: s,
 		}, true
 	}
-	return solutionVehicleImpl{}, false
+	return SolutionVehicle{}, false
 }
 
 func (s *solutionImpl) Copy() Solution {
@@ -834,7 +834,7 @@ func (s *solutionImpl) newVehicle(
 	modelVehicle ModelVehicle,
 ) (SolutionVehicle, error) {
 	if modelVehicle == nil {
-		return nil, fmt.Errorf("modelVehicle is nil")
+		return SolutionVehicle{}, fmt.Errorf("modelVehicle is nil")
 	}
 
 	model := s.model.(*modelImpl)
@@ -858,11 +858,11 @@ func (s *solutionImpl) newVehicle(
 		modelVehicle.Last().Index(),
 	)
 	s.vehicleIndices = append(s.vehicleIndices, modelVehicle.Index())
-	s.vehicles = append(s.vehicles, solutionVehicleImpl{
+	s.vehicles = append(s.vehicles, SolutionVehicle{
 		index:    modelVehicle.Index(),
 		solution: s,
 	})
-	s.solutionVehicles = append(s.solutionVehicles, solutionVehicleImpl{
+	s.solutionVehicles = append(s.solutionVehicles, SolutionVehicle{
 		index:    modelVehicle.Index(),
 		solution: s,
 	})
@@ -903,10 +903,10 @@ func (s *solutionImpl) newVehicle(
 
 	constraint, _, err := s.isFeasible(len(s.stop)-2, true)
 	if err != nil {
-		return nil, err
+		return SolutionVehicle{}, err
 	}
 	if constraint != nil {
-		return nil, fmt.Errorf("failed creating new vehicle: %v", constraint)
+		return SolutionVehicle{}, fmt.Errorf("failed creating new vehicle: %v", constraint)
 	}
 
 	return toSolutionVehicle(s, len(s.vehicles)-1), nil
@@ -1083,7 +1083,7 @@ func (s *solutionImpl) BestMove(ctx context.Context, planUnit SolutionPlanUnit)
 		return bestMove
 	}
 
-	solutionVehicle := solutionVehicleImpl{
+	solutionVehicle := SolutionVehicle{
 		index:    -1,
 		solution: s,
 	}
diff --git a/solution_move_stops.go b/solution_move_stops.go
index 44f941a..4e761ef 100644
--- a/solution_move_stops.go
+++ b/solution_move_stops.go
@@ -130,13 +130,13 @@ func (m *solutionMoveStopsImpl) Solution() Solution {
 
 func (m *solutionMoveStopsImpl) Vehicle() SolutionVehicle {
 	if len(m.stopPositions) == 0 {
-		return nil
+		return SolutionVehicle{}
 	}
-	return m.stopPositions[len(m.stopPositions)-1].next().Vehicle()
+	return m.stopPositions[len(m.stopPositions)-1].Next().Vehicle()
 }
 
-func (m *solutionMoveStopsImpl) vehicle() solutionVehicleImpl {
-	return m.stopPositions[len(m.stopPositions)-1].next().vehicle()
+func (m *solutionMoveStopsImpl) vehicle() SolutionVehicle {
+	return m.stopPositions[len(m.stopPositions)-1].Next().vehicle()
 }
 
 func (m *solutionMoveStopsImpl) Next() SolutionStop {
@@ -150,7 +150,7 @@ func (m *solutionMoveStopsImpl) next() (SolutionStop, bool) {
 	if len(m.stopPositions) == 0 {
 		return SolutionStop{}, false
 	}
-	return m.stopPositions[len(m.stopPositions)-1].next(), true
+	return m.stopPositions[len(m.stopPositions)-1].Next(), true
 }
 
 func (m *solutionMoveStopsImpl) Previous() SolutionStop {
@@ -165,7 +165,7 @@ func (m *solutionMoveStopsImpl) previous() (SolutionStop, bool) {
 	if len(m.stopPositions) == 0 {
 		return SolutionStop{}, false
 	}
-	return m.stopPositions[0].previous(), true
+	return m.stopPositions[0].Previous(), true
 }
 
 func (m *solutionMoveStopsImpl) Execute(_ context.Context) (bool, error) {
@@ -198,7 +198,7 @@ func (m *solutionMoveStopsImpl) Execute(_ context.Context) (bool, error) {
 		}
 
 		for _, position := range m.stopPositions {
-			position.stop().detach()
+			position.Stop().detach()
 		}
 
 		constraint, _, err := m.planUnit.solution().isFeasible(startPropagate, true)
@@ -224,9 +224,9 @@ func (m *solutionMoveStopsImpl) attach() (int, error) {
 	startPropagate := -1
 	for i := len(m.stopPositions) - 1; i >= 0; i-- {
 		stopPosition := m.stopPositions[i]
-		m.planUnit.solutionStops[i] = stopPosition.stop()
-		beforeStop := stopPosition.next()
-		if stopPosition.stop().IsPlanned() {
+		m.planUnit.solutionStops[i] = stopPosition.Stop()
+		beforeStop := stopPosition.Next()
+		if stopPosition.Stop().IsPlanned() {
 			return -1, fmt.Errorf(
 				"stop %v is already planned",
 				stopPosition.Stop(),
@@ -238,7 +238,7 @@ func (m *solutionMoveStopsImpl) attach() (int, error) {
 				beforeStop,
 			)
 		}
-		startPropagate = stopPosition.stop().attach(
+		startPropagate = stopPosition.Stop().attach(
 			beforeStop.PreviousIndex(),
 		)
 	}
@@ -321,7 +321,7 @@ func (m *solutionMoveStopsImpl) TakeBest(that SolutionMove) SolutionMove {
 func (m *solutionMoveStopsImpl) deltaStopTravelDurationValue(
 	vehicleType ModelVehicleType,
 ) float64 {
-	if len(m.stopPositions) == 0 || m.stopPositions[0].stop().IsPlanned() {
+	if len(m.stopPositions) == 0 || m.stopPositions[0].Stop().IsPlanned() {
 		return 0
 	}
 	deltaStopDurationValue := 0.0
@@ -329,12 +329,12 @@ func (m *solutionMoveStopsImpl) deltaStopTravelDurationValue(
 	vehicleTravelDuration := vehicleType.TravelDurationExpression()
 	vehicleDuration := vehicleType.DurationExpression()
 	for _, stopPosition := range m.stopPositions {
-		modelStop := stopPosition.stop().ModelStop()
-		nextStop := stopPosition.next().ModelStop()
-		previousStop := stopPosition.previous().ModelStop()
-		if stopPosition.next().IsPlanned() {
-			deltaStopDurationValue -= stopPosition.next().DurationValue()
-			travelDuration -= stopPosition.next().TravelDurationValue()
+		modelStop := stopPosition.Stop().ModelStop()
+		nextStop := stopPosition.Next().ModelStop()
+		previousStop := stopPosition.Previous().ModelStop()
+		if stopPosition.Next().IsPlanned() {
+			deltaStopDurationValue -= stopPosition.Next().DurationValue()
+			travelDuration -= stopPosition.Next().TravelDurationValue()
 			travelDuration += vehicleTravelDuration.Value(
 				vehicleType,
 				modelStop,
@@ -361,7 +361,7 @@ func (m *solutionMoveStopsImpl) deltaStopTravelDurationValue(
 }
 
 func (m *solutionMoveStopsImpl) deltaTravelDurationValue() float64 {
-	if len(m.stopPositions) == 0 || m.stopPositions[0].stop().IsPlanned() {
+	if len(m.stopPositions) == 0 || m.stopPositions[0].Stop().IsPlanned() {
 		return 0
 	}
 
@@ -373,7 +373,7 @@ func (m *solutionMoveStopsImpl) deltaTravelDurationValue() float64 {
 
 	if isDependentOnTime {
 		if len(m.stopPositions) == 1 {
-			solutionStop := m.stopPositions[0].stop()
+			solutionStop := m.stopPositions[0].Stop()
 			previousStop, _ := m.previous()
 			departure := previousStop.EndValue()
 			fromDuration, _, _, _ := vehicleType.TemporalValues(
@@ -422,18 +422,18 @@ func (m *solutionMoveStopsImpl) deltaTravelDurationValue() float64 {
 	travelDuration := 0.0
 
 	for _, stopPosition := range m.stopPositions {
-		modelStop := stopPosition.stop().ModelStop()
-		if stopPosition.next().IsPlanned() {
-			travelDuration -= stopPosition.next().TravelDurationValue()
+		modelStop := stopPosition.Stop().ModelStop()
+		if stopPosition.Next().IsPlanned() {
+			travelDuration -= stopPosition.Next().TravelDurationValue()
 			travelDuration += vehicleType.TravelDurationExpression().Value(
 				vehicleType,
 				modelStop,
-				stopPosition.next().ModelStop(),
+				stopPosition.Next().ModelStop(),
 			)
 		}
 		travelDuration += vehicleType.TravelDurationExpression().Value(
 			vehicleType,
-			stopPosition.previous().ModelStop(),
+			stopPosition.Previous().ModelStop(),
 			modelStop,
 		)
 	}
@@ -493,7 +493,7 @@ func newMoveStops(
 		)
 	}
 
-	vehicle := stopPositions[0].previous().vehicle()
+	vehicle := stopPositions[0].Previous().vehicle()
 
 	lastPlannedPreviousStop := stopPositions[0].Previous()
 
@@ -518,65 +518,65 @@ func newMoveStops(
 				)
 		}
 
-		if stopPosition.previous().IsPlanned() {
-			if stopPosition.previous().Position() < position {
+		if stopPosition.Previous().IsPlanned() {
+			if stopPosition.Previous().Position() < position {
 				return nil,
 					fmt.Errorf("previous stop %s of stop position %v is planned at position %v,"+
 						" which is before or at the last planned previous stop %s at position %v",
-						stopPosition.previous().ModelStop().ID(),
+						stopPosition.Previous().ModelStop().ID(),
 						index,
-						stopPosition.previous().Position(),
+						stopPosition.Previous().Position(),
 						lastPlannedPreviousStop.ModelStop().ID(),
 						lastPlannedPreviousStop.Position(),
 					)
 			}
-			position = stopPosition.previous().Position()
+			position = stopPosition.Previous().Position()
 
-			lastPlannedPreviousStop = stopPosition.previous()
+			lastPlannedPreviousStop = stopPosition.Previous()
 		}
-		if stopPosition.next().IsPlanned() {
-			if stopPosition.next().Position() < position {
+		if stopPosition.Next().IsPlanned() {
+			if stopPosition.Next().Position() < position {
 				return nil,
 					fmt.Errorf("next stop %s of stop position %v is planned at position %v,"+
 						" which is before or at the last planned previous stop %s at position %v",
-						stopPosition.next().ModelStop().ID(),
+						stopPosition.Next().ModelStop().ID(),
 						index,
-						stopPosition.next().Position(),
+						stopPosition.Next().Position(),
 						lastPlannedPreviousStop.ModelStop().ID(),
 						lastPlannedPreviousStop.Position(),
 					)
 			}
-			position = stopPosition.next().Position()
+			position = stopPosition.Next().Position()
 		}
-		if stopPosition.next().IsPlanned() && !stopPosition.previous().IsPlanned() {
-			if lastPlannedPreviousStop.Position() != stopPosition.next().Position()-1 {
+		if stopPosition.Next().IsPlanned() && !stopPosition.Previous().IsPlanned() {
+			if lastPlannedPreviousStop.Position() != stopPosition.Next().Position()-1 {
 				return nil,
 					fmt.Errorf("stop positions are not allowed, planned previous stop %v is not adjacent"+
 						" to the planned next stop %v of the next stop position",
 						lastPlannedPreviousStop.ModelStop().ID(),
-						stopPosition.next().ModelStop().ID(),
+						stopPosition.Next().ModelStop().ID(),
 					)
 			}
 		}
 
-		if stopPosition.next().IsPlanned() && stopPosition.previous().IsPlanned() {
-			if stopPosition.next().Position() != stopPosition.previous().Position()+1 {
+		if stopPosition.Next().IsPlanned() && stopPosition.Previous().IsPlanned() {
+			if stopPosition.Next().Position() != stopPosition.Previous().Position()+1 {
 				return nil,
 					fmt.Errorf("stop positions are not allowed, planned previous stop %v is not adjacent"+
 						" to the planned next stop %v of stop position %v",
-						stopPosition.previous().ModelStop().ID(),
-						stopPosition.next().ModelStop().ID(),
+						stopPosition.Previous().ModelStop().ID(),
+						stopPosition.Next().ModelStop().ID(),
 						index,
 					)
 			}
 		}
 
-		if !stopPosition.previous().IsPlanned() {
-			if stopPositions[index-1].Stop() != stopPosition.previous() {
+		if !stopPosition.Previous().IsPlanned() {
+			if stopPositions[index-1].Stop() != stopPosition.Previous() {
 				return nil,
 					fmt.Errorf("the previous stop %s of stop position %v"+
 						" must be the stop %s of the previous stop position %v if it is unplanned",
-						stopPosition.previous().ModelStop().ID(),
+						stopPosition.Previous().ModelStop().ID(),
 						index,
 						stopPositions[index-1].Stop().ModelStop().ID(),
 						index-1,
@@ -584,12 +584,12 @@ func newMoveStops(
 			}
 		}
 
-		if !stopPosition.next().IsPlanned() {
-			if stopPositions[index+1].Stop() != stopPosition.next() {
+		if !stopPosition.Next().IsPlanned() {
+			if stopPositions[index+1].Stop() != stopPosition.Next() {
 				return nil,
 					fmt.Errorf("the next stop %s of stop position %v"+
 						" must be the stop %s of the next stop position %v if it is unplanned",
-						stopPosition.next().ModelStop().ID(),
+						stopPosition.Next().ModelStop().ID(),
 						index,
 						stopPositions[index+1].Stop().ModelStop().ID(),
 						index+1,
@@ -597,23 +597,23 @@ func newMoveStops(
 			}
 		}
 
-		if stopPosition.previous().IsPlanned() && stopPosition.previous().vehicle().index != vehicle.index {
+		if stopPosition.Previous().IsPlanned() && stopPosition.Previous().vehicle().index != vehicle.index {
 			return nil,
 				fmt.Errorf(
 					"planned previous stop %v of stop position %v vehicle mismatch: %v != %v",
-					stopPosition.previous().ModelStop().ID(),
+					stopPosition.Previous().ModelStop().ID(),
 					index,
-					stopPosition.previous().vehicle().ModelVehicle().ID(),
+					stopPosition.Previous().vehicle().ModelVehicle().ID(),
 					vehicle.ModelVehicle().ID(),
 				)
 		}
-		if stopPosition.next().IsPlanned() && stopPosition.next().vehicle().index != vehicle.index {
+		if stopPosition.Next().IsPlanned() && stopPosition.Next().vehicle().index != vehicle.index {
 			return nil,
 				fmt.Errorf(
 					"planned next stop %v of stop position %v vehicle mismatch: %v != %v",
-					stopPosition.next().ModelStop().ID(),
+					stopPosition.Next().ModelStop().ID(),
 					index,
-					stopPosition.next().vehicle().ModelVehicle().ID(),
+					stopPosition.Next().vehicle().ModelVehicle().ID(),
 					vehicle.ModelVehicle().ID(),
 				)
 		}
diff --git a/solution_move_stops_generator.go b/solution_move_stops_generator.go
index fc0cefb..76e5bbb 100644
--- a/solution_move_stops_generator.go
+++ b/solution_move_stops_generator.go
@@ -21,7 +21,7 @@ import (
 //		) {
 //	 }
 func SolutionMoveStopsGeneratorChannel(
-	vehicle solutionVehicleImpl,
+	vehicle SolutionVehicle,
 	planUnit *solutionPlanStopsUnitImpl,
 	quit <-chan struct{},
 	stops SolutionStops,
@@ -64,7 +64,7 @@ func SolutionMoveStopsGeneratorChannelTest(
 	preAllocatedMoveContainer *PreAllocatedMoveContainer,
 ) chan SolutionMoveStops {
 	return SolutionMoveStopsGeneratorChannel(
-		vehicle.(solutionVehicleImpl),
+		vehicle,
 		planUnit.(*solutionPlanStopsUnitImpl),
 		quit,
 		stops,
@@ -82,7 +82,7 @@ func SolutionMoveStopsGeneratorTest(
 	shouldStop func() bool,
 ) {
 	SolutionMoveStopsGenerator(
-		vehicle.(solutionVehicleImpl),
+		vehicle,
 		planUnit.(*solutionPlanStopsUnitImpl),
 		yield,
 		stops,
@@ -94,7 +94,7 @@ func SolutionMoveStopsGeneratorTest(
 // SolutionMoveStopsGenerator generates all possible moves for a given vehicle and
 // plan unit. The function yield is called for each solutionMoveStopsImpl.
 func SolutionMoveStopsGenerator(
-	vehicle solutionVehicleImpl,
+	vehicle SolutionVehicle,
 	planUnit *solutionPlanStopsUnitImpl,
 	yield func(move SolutionMoveStops),
 	stops SolutionStops,
@@ -221,12 +221,12 @@ func generate(
 				stopPositions[positionIdx-1].nextStopIndex = stopPositions[positionIdx].stopIndex
 			} else {
 				stopPositions[positionIdx-1].nextStopIndex = target[combination[positionIdx-1]].index
-				if mustBeNeighbours(model, stopPositions[positionIdx-1].stop(), stopPositions[positionIdx].stop()) {
+				if mustBeNeighbours(model, stopPositions[positionIdx-1].Stop(), stopPositions[positionIdx].Stop()) {
 					break
 				}
 			}
 
-			if isNotAllowed(model, stopPositions[positionIdx-1].stop(), stopPositions[positionIdx-1].next()) {
+			if isNotAllowed(model, stopPositions[positionIdx-1].Stop(), stopPositions[positionIdx-1].Next()) {
 				combination = combination[:positionIdx]
 				if stopPositions[positionIdx-1].nextStopIndex != stopPositions[positionIdx].previousStopIndex {
 					break
@@ -235,7 +235,7 @@ func generate(
 			}
 		}
 
-		if isNotAllowed(model, stopPositions[positionIdx].previous(), stopPositions[positionIdx].stop()) {
+		if isNotAllowed(model, stopPositions[positionIdx].Previous(), stopPositions[positionIdx].Stop()) {
 			combination = combination[:positionIdx]
 			continue
 		}
diff --git a/solution_move_test.go b/solution_move_test.go
index d8f000d..770bc68 100644
--- a/solution_move_test.go
+++ b/solution_move_test.go
@@ -101,7 +101,7 @@ func TestVehicleBestMoveSinglePlanUnit(t *testing.T) {
 
 	solutionVehicle := solution.SolutionVehicle(model.Vehicle(0))
 
-	if solutionVehicle == nil {
+	if solutionVehicle.IsZero() {
 		t.Error("solutionVehicle is nil")
 	}
 
@@ -188,7 +188,7 @@ func TestVehicleBestMoveSequencePlanUnit(t *testing.T) {
 
 	solutionVehicle := solution.SolutionVehicle(model.Vehicle(0))
 
-	if solutionVehicle == nil {
+	if solutionVehicle.IsZero() {
 		t.Error("solutionVehicle is nil")
 	}
 
diff --git a/solution_stop.go b/solution_stop.go
index d77c033..414195f 100644
--- a/solution_stop.go
+++ b/solution_stop.go
@@ -279,11 +279,11 @@ func (v SolutionStop) Vehicle() SolutionVehicle {
 	if v.solution.next[v.index] == v.solution.previous[v.index] {
 		panic("cannot get route of unplanned visit")
 	}
-	return v.solution.solutionVehicles[v.solution.inVehicle[v.index]]
+	return v.vehicle()
 }
 
-func (v SolutionStop) vehicle() solutionVehicleImpl {
-	return solutionVehicleImpl{
+func (v SolutionStop) vehicle() SolutionVehicle {
+	return SolutionVehicle{
 		index:    v.solution.inVehicle[v.index],
 		solution: v.solution,
 	}
diff --git a/solution_stop_generator.go b/solution_stop_generator.go
index 5e53305..d839189 100644
--- a/solution_stop_generator.go
+++ b/solution_stop_generator.go
@@ -58,9 +58,9 @@ func newSolutionStopGenerator(
 	startAtFirst bool,
 	endAtLast bool,
 ) *solutionStopGeneratorImpl {
-	nextStop := move.vehicle().first()
+	nextStop := move.Vehicle().First()
 	if !startAtFirst {
-		nextStop = move.stopPositions[0].previous()
+		nextStop = move.stopPositions[0].Previous()
 	}
 	solutionStopGenerator := solutionGeneratorPool.Get().(*solutionStopGeneratorImpl)
 	solutionStopGenerator.stopPositions = solutionStopGenerator.stopPositions[:0]
@@ -102,9 +102,9 @@ func (s *solutionStopGeneratorImpl) next() (SolutionStop, bool) {
 	returnStop := s.nextStop
 
 	if s.startAtFirst {
-		if s.nextStop == s.stopPositions[s.activeStopPositionIndex].previous() {
+		if s.nextStop == s.stopPositions[s.activeStopPositionIndex].Previous() {
 			s.startAtFirst = false
-			s.nextStop = s.stopPositions[s.activeStopPositionIndex].stop()
+			s.nextStop = s.stopPositions[s.activeStopPositionIndex].Stop()
 		} else {
 			s.nextStop = s.nextStop.Next()
 		}
@@ -112,18 +112,18 @@ func (s *solutionStopGeneratorImpl) next() (SolutionStop, bool) {
 	}
 
 	if s.activeStopPositionIndex < len(s.stopPositions) {
-		if s.nextStop == s.stopPositions[s.activeStopPositionIndex].stop() {
-			s.nextStop = s.stopPositions[s.activeStopPositionIndex].next()
+		if s.nextStop == s.stopPositions[s.activeStopPositionIndex].Stop() {
+			s.nextStop = s.stopPositions[s.activeStopPositionIndex].Next()
 			s.activeStopPositionIndex++
 			return returnStop, true
 		}
-		if s.nextStop == s.stopPositions[s.activeStopPositionIndex].previous() {
-			s.nextStop = s.stopPositions[s.activeStopPositionIndex].stop()
+		if s.nextStop == s.stopPositions[s.activeStopPositionIndex].Previous() {
+			s.nextStop = s.stopPositions[s.activeStopPositionIndex].Stop()
 			s.activeStopPositionIndex++
 			return returnStop, true
 		}
 		if !s.nextStop.IsPlanned() {
-			s.nextStop = s.stopPositions[s.activeStopPositionIndex-1].next()
+			s.nextStop = s.stopPositions[s.activeStopPositionIndex-1].Next()
 		} else {
 			if s.nextStop.IsLast() {
 				s.endReached = true
@@ -136,7 +136,7 @@ func (s *solutionStopGeneratorImpl) next() (SolutionStop, bool) {
 	}
 
 	if !s.nextStop.IsPlanned() {
-		s.nextStop = s.stopPositions[s.activeStopPositionIndex-1].next()
+		s.nextStop = s.stopPositions[s.activeStopPositionIndex-1].Next()
 		return returnStop, true
 	}
 
diff --git a/solution_stop_position.go b/solution_stop_position.go
index 6f12fb3..7393b82 100644
--- a/solution_stop_position.go
+++ b/solution_stop_position.go
@@ -67,12 +67,12 @@ func newStopPosition(
 
 func (v StopPosition) String() string {
 	return fmt.Sprintf("stopPosition{%s[%v]->%s[%v]->%s[%v]",
-		v.previous().ModelStop().ID(),
-		v.previous().Index(),
-		v.stop().ModelStop().ID(),
-		v.stop().Index(),
-		v.next().ModelStop().ID(),
-		v.next().Index(),
+		v.Previous().ModelStop().ID(),
+		v.Previous().Index(),
+		v.Stop().ModelStop().ID(),
+		v.Stop().Index(),
+		v.Next().ModelStop().ID(),
+		v.Next().Index(),
 	)
 }
 
@@ -80,38 +80,26 @@ func (v StopPosition) String() string {
 // involving the stop position is executed. It's worth noting that
 // the previous stop may not have been planned yet.
 func (v StopPosition) Previous() SolutionStop {
-	return v.previous()
-}
-
-// Next denotes the upcoming stop's next stop if the associated move
-// involving the stop position is executed. It's worth noting that
-// the next stop may not have been planned yet.
-func (v StopPosition) Next() SolutionStop {
-	return v.next()
-}
-
-// Stop returns the stop which is not yet part of the solution. This stop
-// is not planned yet if the move where the invoking stop position belongs
-// to, has not been executed yet.
-func (v StopPosition) Stop() SolutionStop {
-	return v.stop()
-}
-
-func (v StopPosition) previous() SolutionStop {
 	return SolutionStop{
 		index:    v.previousStopIndex,
 		solution: v.solution,
 	}
 }
 
-func (v StopPosition) next() SolutionStop {
+// Next denotes the upcoming stop's next stop if the associated move
+// involving the stop position is executed. It's worth noting that
+// the next stop may not have been planned yet.
+func (v StopPosition) Next() SolutionStop {
 	return SolutionStop{
 		index:    v.nextStopIndex,
 		solution: v.solution,
 	}
 }
 
-func (v StopPosition) stop() SolutionStop {
+// Stop returns the stop which is not yet part of the solution. This stop
+// is not planned yet if the move where the invoking stop position belongs
+// to, has not been executed yet.
+func (v StopPosition) Stop() SolutionStop {
 	return SolutionStop{
 		index:    v.stopIndex,
 		solution: v.solution,
diff --git a/solution_vehicle.go b/solution_vehicle.go
index 9324be5..1647810 100644
--- a/solution_vehicle.go
+++ b/solution_vehicle.go
@@ -14,94 +14,25 @@ import (
 )
 
 // SolutionVehicle is a vehicle in a solution.
-type SolutionVehicle interface {
-	// FirstMove creates a move that adds the given plan unit to the
-	// vehicle after the first solution stop of the vehicle. The move is
-	// first feasible move after the first solution stop based on the
-	// estimates of the constraint, this move is not necessarily executable.
-	FirstMove(SolutionPlanUnit) (SolutionMove, error)
-
-	// BestMove returns the best move for the given solution plan unit on
-	// the invoking vehicle. The best move is the move that has the lowest
-	// score. If there are no moves available for the given solution plan
-	// unit, a move is returned which is not executable, SolutionMoveStops.IsExecutable.
-	BestMove(context.Context, SolutionPlanUnit) SolutionMove
-
-	// Duration returns the duration of the vehicle. The duration is the
-	// time the vehicle is on the road. The duration is the time between
-	// the start time and the end time.
-	Duration() time.Duration
-	// DurationValue returns the duration value of the vehicle. The duration
-	// value is the value of the duration of the vehicle. The duration value
-	// is the value in model duration units.
-	DurationValue() float64
-
-	// End returns the end time of the vehicle. The end time is the time
-	// the vehicle ends at the end stop.
-	End() time.Time
-	// EndValue returns the end value of the vehicle. The end value is the
-	// value of the end of the last stop. The end value is the value in
-	// model duration units since the model epoch.
-	EndValue() float64
-
-	// First returns the first stop of the vehicle. The first stop is the
-	// start stop.
-	First() SolutionStop
-
-	// Index returns the index of the vehicle in the solution.
-	Index() int
-	// IsEmpty returns true if the vehicle is empty, false otherwise. A
-	// vehicle is empty if it does not have any stops. The start and end
-	// stops are not considered.
-	IsEmpty() bool
-
-	// Last returns the last stop of the vehicle. The last stop is the end
-	// stop.
-	Last() SolutionStop
-
-	// ModelVehicle returns the modeled vehicle type of the vehicle.
-	ModelVehicle() ModelVehicle
-
-	// NumberOfStops returns the number of stops in the vehicle. The start
-	// and end stops are not considered.
-	NumberOfStops() int
-
-	// SolutionStops returns the stops in the vehicle. The start and end
-	// stops are included in the returned stops.
-	SolutionStops() SolutionStops
-	// Start returns the start time of the vehicle. The start time is
-	// the time the vehicle starts at the start stop, it has been set
-	// in the factory method of the vehicle Solution.NewVehicle.
-	Start() time.Time
-	// StartValue returns the start value of the vehicle. The start value
-	// is the value of the start of the first stop. The start value is
-	// the value in model duration units since the model epoch.
-	StartValue() float64
-
-	// Unplan removes all stops from the vehicle. The start and end stops
-	// are not removed. Fixed stops are not removed.
-	Unplan() (bool, error)
+type SolutionVehicle struct {
+	solution *solutionImpl
+	index    int
 }
 
 // SolutionVehicles is a slice of solution vehicles.
 type SolutionVehicles []SolutionVehicle
 
-type solutionVehicleImpl struct {
-	solution *solutionImpl
-	index    int
-}
-
 func toSolutionVehicle(
 	solution Solution,
 	index int,
 ) SolutionVehicle {
-	return solutionVehicleImpl{
+	return SolutionVehicle{
 		index:    index,
 		solution: solution.(*solutionImpl),
 	}
 }
 
-func (v solutionVehicleImpl) firstMovePlanStopsUnit(
+func (v SolutionVehicle) firstMovePlanStopsUnit(
 	planUnit *solutionPlanStopsUnitImpl,
 	preAllocatedMoveContainer *PreAllocatedMoveContainer,
 ) (SolutionMove, error) {
@@ -133,7 +64,7 @@ func (v solutionVehicleImpl) firstMovePlanStopsUnit(
 	return bestMove, nil
 }
 
-func (v solutionVehicleImpl) firstMovePlanUnitsUnit(
+func (v SolutionVehicle) firstMovePlanUnitsUnit(
 	planUnit *solutionPlanUnitsUnitImpl,
 ) (SolutionMove, error) {
 	if planUnit.ModelPlanUnitsUnit().PlanOneOf() {
@@ -142,7 +73,7 @@ func (v solutionVehicleImpl) firstMovePlanUnitsUnit(
 	return v.firstMovePlanAllUnit(planUnit)
 }
 
-func (v solutionVehicleImpl) firstMovePlanOneOfUnit(
+func (v SolutionVehicle) firstMovePlanOneOfUnit(
 	planUnit *solutionPlanUnitsUnitImpl,
 ) (SolutionMove, error) {
 	planUnits := common.Shuffle(
@@ -161,7 +92,7 @@ func (v solutionVehicleImpl) firstMovePlanOneOfUnit(
 	return NotExecutableMove, nil
 }
 
-func (v solutionVehicleImpl) firstMovePlanAllUnit(
+func (v SolutionVehicle) firstMovePlanAllUnit(
 	planUnit *solutionPlanUnitsUnitImpl,
 ) (SolutionMove, error) {
 	planUnits := common.Shuffle(
@@ -235,7 +166,7 @@ func updateMoveInPlace(move SolutionMoveStops, moveContainer moveContainer) {
 	move.(*solutionMoveStopsImpl).value = moveContainer.value
 }
 
-func (v solutionVehicleImpl) bestMovePlanSingleStop(
+func (v SolutionVehicle) bestMovePlanSingleStop(
 	_ context.Context,
 	planUnit *solutionPlanStopsUnitImpl,
 	preAllocatedMoveContainer *PreAllocatedMoveContainer,
@@ -248,7 +179,7 @@ func (v solutionVehicleImpl) bestMovePlanSingleStop(
 		move.(*solutionMoveStopsImpl).stopPositions,
 		StopPosition{},
 	)
-	stop := v.first()
+	stop := v.First()
 
 	movesPtr := moveContainerPool.Get().(*[]moveContainer)
 	moves := *movesPtr
@@ -345,7 +276,7 @@ func (v solutionVehicleImpl) bestMovePlanSingleStop(
 	return move
 }
 
-func (v solutionVehicleImpl) bestMoveSequence(
+func (v SolutionVehicle) bestMoveSequence(
 	_ context.Context,
 	planUnit *solutionPlanStopsUnitImpl,
 	sequence SolutionStops,
@@ -379,7 +310,7 @@ func (v solutionVehicleImpl) bestMoveSequence(
 	return bestMove
 }
 
-func (v solutionVehicleImpl) bestMovePlanMultipleStops(
+func (v SolutionVehicle) bestMovePlanMultipleStops(
 	ctx context.Context,
 	planUnit *solutionPlanStopsUnitImpl,
 	preAllocatedMoveContainer *PreAllocatedMoveContainer,
@@ -394,7 +325,7 @@ func (v solutionVehicleImpl) bestMovePlanMultipleStops(
 	return bestMove
 }
 
-func (v solutionVehicleImpl) bestMovePlanStopsUnit(
+func (v SolutionVehicle) bestMovePlanStopsUnit(
 	ctx context.Context,
 	planUnit *solutionPlanStopsUnitImpl,
 	preAllocatedMoveContainer *PreAllocatedMoveContainer,
@@ -406,7 +337,7 @@ func (v solutionVehicleImpl) bestMovePlanStopsUnit(
 	return v.bestMovePlanMultipleStops(ctx, planUnit, preAllocatedMoveContainer)
 }
 
-func (v solutionVehicleImpl) bestMovePlanUnitsUnit(
+func (v SolutionVehicle) bestMovePlanUnitsUnit(
 	ctx context.Context,
 	planUnit *solutionPlanUnitsUnitImpl,
 ) SolutionMove {
@@ -416,7 +347,7 @@ func (v solutionVehicleImpl) bestMovePlanUnitsUnit(
 	return v.bestMovePlanAllUnit(ctx, planUnit)
 }
 
-func (v solutionVehicleImpl) bestMovePlanOneOfUnit(
+func (v SolutionVehicle) bestMovePlanOneOfUnit(
 	ctx context.Context,
 	planUnit *solutionPlanUnitsUnitImpl,
 ) SolutionMove {
@@ -439,7 +370,7 @@ func revertMoves(moves SolutionMoves) (bool, error) {
 	return true, nil
 }
 
-func (v solutionVehicleImpl) bestMovePlanAllUnit(
+func (v SolutionVehicle) bestMovePlanAllUnit(
 	ctx context.Context,
 	planUnit *solutionPlanUnitsUnitImpl,
 ) SolutionMove {
@@ -484,7 +415,11 @@ func (v solutionVehicleImpl) bestMovePlanAllUnit(
 	return newSolutionMoveUnits(planUnit, moves)
 }
 
-func (v solutionVehicleImpl) FirstMove(
+// FirstMove creates a move that adds the given plan unit to the
+// vehicle after the first solution stop of the vehicle. The move is
+// first feasible move after the first solution stop based on the
+// estimates of the constraint, this move is not necessarily executable.
+func (v SolutionVehicle) FirstMove(
 	planUnit SolutionPlanUnit,
 ) (SolutionMove, error) {
 	switch planUnit.(type) {
@@ -497,7 +432,11 @@ func (v solutionVehicleImpl) FirstMove(
 	return NotExecutableMove, nil
 }
 
-func (v solutionVehicleImpl) BestMove(
+// BestMove returns the best move for the given solution plan unit on
+// the invoking vehicle. The best move is the move that has the lowest
+// score. If there are no moves available for the given solution plan
+// unit, a move is returned which is not executable, SolutionMoveStops.IsExecutable.
+func (v SolutionVehicle) BestMove(
 	ctx context.Context,
 	planUnit SolutionPlanUnit,
 ) SolutionMove {
@@ -508,7 +447,7 @@ func (v solutionVehicleImpl) BestMove(
 	return v.bestMove(ctx, planUnit, allocations)
 }
 
-func (v solutionVehicleImpl) bestMove(
+func (v SolutionVehicle) bestMove(
 	ctx context.Context,
 	planUnit SolutionPlanUnit,
 	sharedMoveContainer *PreAllocatedMoveContainer,
@@ -530,72 +469,86 @@ func (v solutionVehicleImpl) bestMove(
 	}
 }
 
-func (v solutionVehicleImpl) IsEmpty() bool {
-	return v.last().Position() == 1
+// IsEmpty returns true if the vehicle is empty, false otherwise. A
+// vehicle is empty if it does not have any stops. The start and end
+// stops are not considered.
+func (v SolutionVehicle) IsEmpty() bool {
+	return v.Last().Position() == 1
 }
 
-func (v solutionVehicleImpl) NumberOfStops() int {
-	return v.last().Position() - 1
+// NumberOfStops returns the number of stops in the vehicle. The start
+// and end stops are not considered.
+func (v SolutionVehicle) NumberOfStops() int {
+	return v.Last().Position() - 1
 }
 
-func (v solutionVehicleImpl) Index() int {
+// Index returns the index of the vehicle in the solution.
+func (v SolutionVehicle) Index() int {
 	return v.index
 }
 
-func (v solutionVehicleImpl) First() SolutionStop {
-	return v.first()
-}
-
-func (v solutionVehicleImpl) Last() SolutionStop {
-	return v.last()
-}
-
-func (v solutionVehicleImpl) first() SolutionStop {
+// First returns the first stop of the vehicle. The first stop is the
+// start stop.
+func (v SolutionVehicle) First() SolutionStop {
 	return SolutionStop{
 		index:    v.solution.first[v.index],
 		solution: v.solution,
 	}
 }
 
-func (v solutionVehicleImpl) last() SolutionStop {
+// Last returns the last stop of the vehicle. The last stop is the end
+// stop.
+func (v SolutionVehicle) Last() SolutionStop {
 	return SolutionStop{
 		index:    v.solution.last[v.index],
 		solution: v.solution,
 	}
 }
 
-func (v solutionVehicleImpl) DurationValue() float64 {
+// DurationValue returns the duration value of the vehicle. The duration
+// value is the value of the duration of the vehicle. The duration value
+// is the value in model duration units.
+func (v SolutionVehicle) DurationValue() float64 {
 	return v.EndValue() - v.StartValue()
 }
 
-func (v solutionVehicleImpl) Duration() time.Duration {
+// Duration returns the duration of the vehicle. The duration is the
+// time the vehicle is on the road. The duration is the time between
+// the start time and the end time.
+func (v SolutionVehicle) Duration() time.Duration {
 	return v.End().Sub(v.Start())
 }
 
-func (v solutionVehicleImpl) StartValue() float64 {
-	return v.first().StartValue()
+// StartValue returns the start value of the vehicle. The start value
+// is the value of the start of the first stop. The start value is
+// the value in model duration units since the model epoch.
+func (v SolutionVehicle) StartValue() float64 {
+	return v.First().StartValue()
 }
 
-func (v solutionVehicleImpl) Start() time.Time {
-	return v.first().Start()
+// Start returns the start time of the vehicle. The start time is
+// the time the vehicle starts at the start stop, it has been set
+// in the factory method of the vehicle Solution.NewVehicle.
+func (v SolutionVehicle) Start() time.Time {
+	return v.First().Start()
 }
 
-func (v solutionVehicleImpl) EndValue() float64 {
-	return v.last().EndValue()
+// EndValue returns the end value of the vehicle. The end value is the
+// value of the end of the last stop. The end value is the value in
+// model duration units since the model epoch.
+func (v SolutionVehicle) EndValue() float64 {
+	return v.Last().EndValue()
 }
 
-func (v solutionVehicleImpl) End() time.Time {
-	return v.last().End()
+// End returns the end time of the vehicle. The end time is the time
+// the vehicle ends at the end stop.
+func (v SolutionVehicle) End() time.Time {
+	return v.Last().End()
 }
 
-func (v solutionVehicleImpl) Next() SolutionStop {
-	return SolutionStop{
-		index:    v.solution.model.NumberOfStops() + v.index*2 + 1,
-		solution: v.solution,
-	}
-}
-
-func (v solutionVehicleImpl) SolutionStops() SolutionStops {
+// SolutionStops returns the stops in the vehicle. The start and end
+// stops are included in the returned stops.
+func (v SolutionVehicle) SolutionStops() SolutionStops {
 	solutionStops := make(SolutionStops, 0, v.NumberOfStops()+2)
 	solutionStop := v.First()
 	for !solutionStop.IsLast() {
@@ -606,24 +559,16 @@ func (v solutionVehicleImpl) SolutionStops() SolutionStops {
 	return solutionStops
 }
 
-func (v solutionVehicleImpl) solutionStops() []SolutionStop {
-	solutionStops := make([]SolutionStop, 0, v.NumberOfStops()+2)
-	solutionStop := v.first()
-	for !solutionStop.IsLast() {
-		solutionStops = append(solutionStops, solutionStop)
-		solutionStop = solutionStop.Next()
-	}
-	solutionStops = append(solutionStops, solutionStop)
-	return solutionStops
-}
-
-func (v solutionVehicleImpl) ModelVehicle() ModelVehicle {
+// ModelVehicle returns the modeled vehicle type of the vehicle.
+func (v SolutionVehicle) ModelVehicle() ModelVehicle {
 	return v.solution.model.Vehicle(v.solution.vehicleIndices[v.index])
 }
 
-func (v solutionVehicleImpl) Unplan() (bool, error) {
+// Unplan removes all stops from the vehicle. The start and end stops
+// are not removed. Fixed stops are not removed.
+func (v SolutionVehicle) Unplan() (bool, error) {
 	// TODO notify observers
-	solutionStops := common.Filter(v.solutionStops(), func(solutionStop SolutionStop) bool {
+	solutionStops := common.Filter(v.SolutionStops(), func(solutionStop SolutionStop) bool {
 		return !solutionStop.IsFixed()
 	})
 	if len(solutionStops) == 0 {
@@ -659,8 +604,8 @@ func (v solutionVehicleImpl) Unplan() (bool, error) {
 	if constraint != nil {
 		for i := len(stopPositions) - 1; i >= 0; i-- {
 			stopPosition := stopPositions[i]
-			beforeStop := stopPosition.next()
-			stopPosition.stop().attach(
+			beforeStop := stopPosition.Next()
+			stopPosition.Stop().attach(
 				beforeStop.PreviousIndex(),
 			)
 		}
@@ -681,3 +626,9 @@ func (v solutionVehicleImpl) Unplan() (bool, error) {
 
 	return true, nil
 }
+
+// IsZero returns true if the solution vehicle is the zero value.
+// In this case it is not safe to use the solution vehicle.
+func (v SolutionVehicle) IsZero() bool {
+	return v.solution == nil && v.index == 0
+}
diff --git a/solve_operator_unplan_vehicles.go b/solve_operator_unplan_vehicles.go
index 9cf6e05..1566807 100644
--- a/solve_operator_unplan_vehicles.go
+++ b/solve_operator_unplan_vehicles.go
@@ -74,7 +74,7 @@ func (d *solveOperatorUnPlanVehiclesImpl) Execute(
 
 	vehicles := common.Filter(
 		workSolution.vehicles,
-		func(vehicle solutionVehicleImpl) bool {
+		func(vehicle SolutionVehicle) bool {
 			return vehicle.NumberOfStops() > 0
 		},
 	)
@@ -82,7 +82,7 @@ func (d *solveOperatorUnPlanVehiclesImpl) Execute(
 	if len(vehicles) == 0 {
 		return nil
 	}
-	weights := common.Map(vehicles, func(vehicle solutionVehicleImpl) float64 {
+	weights := common.Map(vehicles, func(vehicle SolutionVehicle) float64 {
 		return 1.0 + float64(workSolution.Model().NumberOfStops()-vehicle.NumberOfStops())
 	})
 	alias, err := common.NewAlias(weights)