Platoon properties and properties of platooning vehicles

A platoon consists of a lead vehicle and one or multiple following vehicles. The platoon is formed by vehicles closung up when the conditions for closing up and forming the platoon are met (Closing up and forming a platoon), (Conditions for closing up and forming a platoon). Additional vehicles can become part of the platoon by closing up.

The lead vehicle and subsequent vehicles in the platoon drive synchronously. When accelerating and decelerating, they try to take the gap time into account. The entire platoon uses the same lane. It does not change lanes or drive on the lane of oncoming traffic.

The vehicles in the platoon may take into account vehicle routes or paths of dynamic assignment or move throughout the Vissim network without them (Modeling vehicle routing decision, vehicle routes and partial vehicle routes), (Using dynamic assignment).

The lead vehicle or a following vehicle can leave the platoon, for example to follow its route or path. To safely leave the platoon, a following vehicle must increase the distance downstream to its preceding vehicle and upstream to its following vehicle. As soon as the vehicle has left the platoon, the platoon is divided into two platoons (Vehicle leaves platoon).

Acceleration and speed of the trailing vehicles in the platoon are linearly interpolated between the lead vehicle and the last vehicle. Therefore, for trailing vehicles in a platoon, the relationship between acceleration, speed and position is different than for vehicles outside a platoon. For those outside a platoon, this relationship is based on Wiedemann’s car following model. This means acceleration, speed and position of following vehicles in a platoon cannot be consistent.

Closing up and forming a platoon

A vehicle has the following options to become part of a platoon:

• The vehicle closes up to the platoon. When it is close enough, it performs a target braking to reach the calculated safety distance within the platoon. Once the safety distance is reached, the vehicle becomes part of the platoon (Desired speed and safety distance within the platoon).
• The platoon's lead vehicle closes up to a preceding vehicle. When the lead vehicle is close enough and closes up, its preceding vehicle becomes part of the platoon and the new lead vehicle. When the lead vehicle closes up, its subsequent vehicles in the platoon also assume the interaction state of closing up.
• A platoon can also form when a vehicle closes up to a vehicle that is not yet part of a platoon.

In all three cases, the following conditions must be met for closing up and forming the platoon.

Conditions for closing up and forming a platoon

• The number of vehicles in the platoon is less than the number specified in the attribute Maximum number of platoon vehicles.
• The Platooning possible attribute must be selected based on the driving behavior of vehicles already driving within the platoon or becoming part of the platoon.
• There must be no other vehicles between the vehicle closing up and the vehicle being closed up to.
• The vehicle closing up must not want to immediately leave the platoon again. For example, it should not be wanting to change lanes or be braking for an emergency stop.
• The vehicle closing up must be able to perceive the preceding vehicle driving on the same lane.
• The lead vehicle cannot close up to any other vehicle of its platoon, for example when in a roundabout. Forming a closed platooning ring is not possible.
• The vehicle closing up must have a higher desired speed than its preceding vehicle. No vehicle brakes for a vehicle to close up to it and the vehicle closing up does not accelerate.
• The vehicle closing up takes into account the acceleration of the preceding vehicle, if it is > -1.0 m/s².

In addition, the following conditions apply to the vehicle closing up and the vehicle being closed up to:

• The vehicles are assigned the same driving behavior.
• All vehicles use the same lane.
• The vehicles may be on successive links or connectors.
• The preceding vehicle is the interaction target of the vehicle closing up. This is reflected by the attributes interaction target type and interaction target number of the vehicle in the Vehicles in network list.
• The distance between the vehicles is less than the attribute value Maximum platoon approach distance of driving behavior that applies to the vehicles.

None of the following applies for any of the vehicles:

• The vehicle is changing lanes when closing up to a preceding vehicle.
• The vehicle has been assigned a desired lane.
• The vehicle is about to leave the platoon.
• The vehicle is reversing.
• The vehicle is driving on a passing lane of a public transport line or a parking route.
• The vehicle is being controlled via the driving simulator, the COM interface or a Driver model DLL file.

When the conditions for closing up and forming the platoon are met, the vehicle shall close up to the preceding vehicle and apply target braking to reach the calculated safety distance within the platoon (Desired speed and safety distance within the platoon). Once the safety distance is reached, the vehicle becomes part of the platoon. If the platoon's lead vehicle has closed up to its preceding vehicle, the latter becomes the new lead vehicle.

A platoon can also be formed by stationary vehicles if the respective conditions are met. There is no minimum speed for forming a platoon.

In addition, the vehicle that is being closed up to can close up as well, for example, to a signal controller.

The closing up to a preceding vehicle is aborted if one of the conditions, in one of the subsequent time steps, is no longer fulfilled and the closing up has not yet been completed.

Desired lateral position and lane change of vehicles closing up

Vissim Makes the following settings for the vehicle closing up or the vehicle being closed up to:

• The desired lateral position is set to the desired lateral position of the preceding vehicle.
• Non-essential lane changes are deactivated.
• The safety distance is reduced to the calculated safety distance within the platoon.

In the platoon, the following vehicle strives for the desired lateral position of the lead vehicle (Editing the driving behavior parameter Lateral behavior).

Interaction between vehicles and interaction objects

In Vissim a following vehicle within the platoon interacts with the lead vehicle only, not with any other vehicles. However, a following vehicle does react to interaction objects (Editing the driving behavior parameter Following behavior). This enables the following vehicle, for example, to react to a red signal head and leave the platoon (Behavior in conflict situations). The platoon is then split.

In Vissim, the lead vehicle interacts with other vehicles the same way as if it were not part of a platoon. If the interaction is dependent on the vehicle class, the vehicle class of the lead vehicle is taken into account.

Vehicles outside the platoon as well as detectors do not perceive the platoon as an object that belongs together, but react to the platoon's vehicles the same way as to individual vehicles.

When a platoon approaches a preceding platoon and the conditions for closing up to and forming a platoon are met, it can close up to the other platoon. Vissim will then combine the two platoons into one.

Vissim models neither the communication between vehicles and their possible influence on driving behavior nor other platoon-internal dynamics.

Desired speed and safety distance within the platoon

• The lead vehicle attempts to maintain its desired speed according to the car following model selected for the applicable driving behavior. The Maximum platooning desired speed might reduce the lead vehicle’s desired speed.
• Each following vehicle within the platoon follows the lead vehicle in a coordinated manner. It tries to maintain a safety distance to its preceding vehicle. This distance is calculated by Vissim based on:

PlatoonMinClear + vReference * PlatoonFollowUpGapTm

Where:

PlatoonMinClear	Attribute Platooning - minimum clearance
PlatoonFollowUpGapTm	Attribute Platooning - follow-up gap time
vReference	Reference speed: The vehicle providing the reference speed depends on whether distances have to be increased: To leave the platoon, a vehicle must increase the distance downstream to its preceding vehicle and upstream to its following vehicle (F1) (Vehicle leaves platoon). If the platoon is split downstream of the vehicle, the distance downstream to the preceding vehicle must be increased. This is necessary, for example, if a following vehicle has to stop because of a red signal head, a priority rule or a stop sign, while the vehicles further ahead in the platoon were still able to cross. Since increasing the distance affects following vehicles and their safety distances within the platoon, Vissim no longer uses the speed of the platoon's lead vehicle to calculate the distances in this part of the platoon: vReference is the speed of the closest following vehicle in front of which the distance must be increased. In the figure, this is vehicle (F1), whose speed is used to calculate the distances of its trailing vehicles in the platoon. (F1) becomes lead vehicle of this part of the platoon: The lead vehicle of this part of the platoon (F1) increases the gap by decelerating. The deceleration is based either on the vehicle’s desired deceleration or on a calculated necessary deceleration should the vehicle interact with an interaction object. vReference is the speed of the lead vehicle (F) of the platoon, unless greater distances are required within the platoon because a vehicle wishes to leave the platoon or the platoon is split:

If the requirements for the automatic speed limitation in curves are met for the lead vehicle, this vehicle reacts and can reduce its speed (Changing the desired speed). The trailing vehicles react to the lead vehicle. An automatic speed limit in curves cannot cause the dissolution of a platoon.

Lane changes

The platoon does not change lanes. This also applies in the following cases:

• When the Maximum platooning desired speed cannot be reached on the current lane, but could be reached on the adjacent lane. This case may occur, for example, when the vehicle preceding the lead vehicle is driving slower than the Maximum platooning desired speed.

Necessary lane changes remain activated so that the vehicle can leave the platoon and, for example, take an exit to continue its route or to change lanes because it belongs to a class of vehicles for which the lane is blocked further downstream.

Behavior in conflict situations

• If the lead vehicle stops because of a red signal head, a priority rule, a stop sign or a conflict area, the following vehicles in the platoon also stop.
• If the lead vehicle or a trailing vehicle can pass a signal head, a priority rule or a stop sign, but not any additional trailing vehicles of the platoon, Vissim splits the platoon. This is the case when the interaction of a trailing vehicle in the platoon with an interaction object requires an acceleration by the trailing vehicle which is a) less than the acceleration that would apply without the interaction and which b) is also less than the acceleration of the lead vehicle.

Vissim splits the platoon downstream of the vehicle which is able to apply target breaking and stop in front of the network object. This vehicle does not decelerate at the rate of its desired deceleration, but as is necessary to stop at the signal head. The deceleration rate may be higher than that of the desired deceleration. A platoon is not split because of a conflict area.

Behavior in a reduced speed area

• The lead vehicle behaves like an individual vehicle.
• Each following vehicle within the platoon reacts to the behavior of the lead vehicle. However, a following vehicle does not explicitly react to the reduced speed area, but adapts its desired speed in the event that it wants to leave the platoon in the reduced speed area.

Driving behavior at desired speed decisions

• The lead vehicle behaves like an individual vehicle. The desired speed of the lead vehicle becomes the desired speed of the following vehicles. It may be limited by the Maximum platooning desired speed.
• Each following vehicle within the platoon reacts to the behavior of the lead vehicle. However, a following vehicle does not explicitly react to the desired speed decision, but adapts its desired speed in the event that it wants to leave the platoon later on.

Behavior at vehicle routing decisions

• Vehicle routing decisions affect the lead vehicle and any following vehicles. This means that any vehicle can leave the platoon and continue its route.
• When a vehicle traverses the marker of a parking routing decision and follows a parking route, it leaves the platoon. Then Vissim can assign the vehicle a parking space and stop vehicle the vehicle there.

Behavior when driving behavior changes

After a vehicle has become part of the platoon, a different driving behavior may apply. This is the case as soon as the lead vehicle reaches a link with a behavior type that specifies a different driving behavior. The driving behavior may be vehicle class specific.

A different driving behavior may result in:

• If the platoon can no longer be maintained because of the driving behavior attributes of the link, Vissim dissolves the platoon. The vehicles continue driving individually.
• As soon as a vehicle is affected by a driving behavior in which platooning is deactivated, it is removed from the platoon and continues driving as an individual vehicle. This concerns the lead vehicle first, for example when it reaches a corresponding link, and then any of the following vehicles within the platoon as soon as they reach the same link.
• If the lead vehicle reaches a link on which the attribute Maximum number of platoon vehicles limits the number of vehicles in the platoon, Vissim splits the platoon and forms two new platoons where the number of vehicles is equal to or less than the permitted number.
• Changing the attribute Maximum platooning desired speed affects the lead vehicle and thus the platoon.

Vehicle leaves platoon

The lead vehicle or a following vehicle can leave the platoon, for example to follow its route or path on connector or other lane.

A vehicle also leaves the platoon in the following cases:

• When the vehicle leaves the Vissim network
• When the vehicle drives into the part of the network that is simulated mesoscopically (Using mesoscopic simulation)
• When the vehicle is controlled externally (Activating external driver model via Driver Model DLL file)
• When the vehicle no longer perceives a preceding vehicle
• When the attribute Platooning possible no longer applies

If the conditions for forming a platoon are still valid for the following vehicles, they may continue driving as a platoon.

Vissim establishes a safety distance

To ensure that a vehicle (FF) can safely leave the platoon, Vissim increases the distance between this vehicle and its trailing vehicle (F1) in the platton until the safety distances is reached. If the vehicle is a trailing vehicle, i.e. not the lead vehicle, in the platoon, Vissim will also increase the distance to its preceding vehicle (F) in the platoon:

Vissim will only increase these distances if all vehicles in the platoon are either following their vehicle route or their path of dynamic assignment.

Vissim will not increase these distances if some platoon vehicles are following their vehicle route and others are following their path of dynamic assignment or if platoon vehicles are moving freely through the Vissim network without following either a vehicle route or a path. In this case, vehicles simply drive onto the connector, and are immediately removed from the platoon.

Calculation of the safety distance for a part of the platoon

The vehicle (FF) brakes using the deceleration resulting from the interaction with an interaction object or its Desired deceleration set in the attributes of its vehicle type, depending on which of the two deceleration values is smaller.

Vissim calculates the distances from the rear edge of the vehicle (F1), in front of which the distance is increased according to the following scheme:

• The desired distance is calculated for the following vehicle (FL). It is located upstream at the last position of the platoon part that includes vehicle (F1), in front of which the distance is increased:

• Vissim no longer takes into account the speed of the platoon's lead vehicle (F), but the speed of the vehicle (F1) in front of which the distance is increased and has therefore become the lead vehicle of this part of the platoon.
• The last following vehicle in this platoon part (FL) will target brake to reach the desired distance (if it needs to brake more than -1m/s² to do so), or it will accelerate or brake as hard as possible. Its acceleration is limited to the range +-2m/s² around the acceleration of the first vehicle (F1) of this platoon part.
• If there are other vehicles between these two vehicles, Vissim will move them to make their distances exactly the same. Therefore, these distances may have a different value than the calculated safety distance within the platoon.
• There is no jerk limitation.

Conditions for leaving the platoon

The vehicle can only leave the platoon safely if the following conditions are met:

• The distance to its preceding vehicle corresponds to the reduced safety distance.
• The distance to its trailing vehicle corresponds to the trailing vehicle’s reduced safety distance.

As soon as the vehicle has left the platoon and starts driving as an individual vehicle, the settings that apply outside the platoon, for example for desired speed and time gaps, are applied to the vehicle again.

Platoon parts become platoons

As soon as the vehicle has left the platoon, the platoon is split into two:

• The platoon with the original lead vehicle and its following vehicles
• The new platoon with the former trailing vehicles of the vehicle that left the platoon. The former trailing vehicle of the vehicle that left the platoon becomes the lead vehicle of this platoon.

If several vehicles leave the platoon that do not immediately follow each other, several platoons may form, each of which has a lead vehicle.

Simulation restrictions

Platooning is not possible with the following simulation methods:

• Mesoscopic simulation
• in hybrid simulation outside the areas of microscopic simulation
• In the Vissim network, the platoon cannot be used at a vehicle input or in a parking lot with dynamic assignment.