There are a wide range of influences on vehicle speed. They are described in the following. Each of these influences sets an upper bound on the vehicle speed. The actual speed in any given situation is the minimum speed of all influences.
models the maximum speed that a vehicle will travel. It can be thought
of as the maximum speed of the engine or the maximum speed desired by
the driver under any circumstances (possibly these two aspects will be
modelled with separate attributes in the future).
edge/lane speed and speedFactor#
When approaching an edge with a lower speed limit than the current one, a vehicle will slow down so as to stay within the new limit at the time of reaching the new edge.
Each vehicle can be assigned an individual speed multiplier (speedFactor) which then lets it exceed this limit. Since version 1.0.0, vehicles will have a random speedFactor with a deviation of 0.1 and mean of 1.0 which means there will be different desired speeds in the vehicle population by default.
When vehicles are driving freely (unconstrained by other vehicles) they will accelerate until reaching the speed
min(maxSpeed, speedFactor * speedLimit)
Legacy behavior can be achieved by setting option --default.speeddev 0
Since version 0.24.0 it is also possible to define vClass-specific speed limits for every edge.
Car Following Model#
The car following model of a vehicle defines its speed in relation the vehicle ahead. The default model always selects the maximum speed which is safe in the sense of being able to stop in time to avoid a collision.
Acceleration and Deceleration#
All models are subject to constraints in their acceleration an deceleration. By default they will not accelerate stronger then the accel value. The default model plans it's maneuvers so as to stay within the decel value (per second) but other models may interpret this value differently. All models will never brake harder than the emergencyDecel value (which defaults to the same value as decel but may be set independently).
This is just a convention which the available models obey but which could be ignored by custom models.
Some car-following models support the
sigma-attribute which models driver
imperfection. For values above 0, drivers with the default
car-following model will drive slower than would be safe by a random
amount (between [0,
Vehicles approaching an intersection without the right-of-way have to slow down. If the intersection is used by other vehicles which have the right of way, stopping may be necessary until a safe time-window is found. That time windows is based on the same safety assumptions as the car-following model. For the default Krauss-model this means that each vehicle must be able to stop safely even if it's lead vehicle brakes hard to a full stop.
Even if a vehicle has the right-of-way it may need to slow down due to impatient drivers which drive across the intersection. The right-of-way rules at an intersection are defined by the node type-attribute and by traffic lights.
If a vehicle hasn't yet entered the intersection, it will in most cases slow down in response to any other vehicles that have already entered the intersection unless there is an unobstructed waiting place within the intersection (an internal junction) to which it can move. If two vehicles in conflict are within the intersection at the same time, a priority order is established based on their time of entering, their speed, the right of way rules and the state of any traffic lights. This priority order determines which of the vehicles has to slow down and which one may drive unimpeded.
Per default, a vehicle approaching from a minor road slows down until it is 4.5m away from the intersection (even if no prioritized vehicle is nearby). After that it may start to accelerate again if there is a safe gap in traffic. This distance models the visibility and may be configured for each individual connection with the 'visibility' attribute.
Vehicles approaching a junction of type 'zipper' automatically determine a vehicle ordering based on their position and speeds. The may have to slow down in order to follow their determined leader smoothly. By default, zipper merging behavior starts 100m ahead of the junction and this distance may also be configured using the 'visibility' attribute.
Vehicles that pass an intersection may also be subject to reduced speed limits depending on the turning angle.
Vehicles may decide to slow down in order to execute a lane-change maneuver. They may also slow down in order to aid other vehicles with lane-changing. If the lane a vehicle is on does not have a connection to the next edge on a vehicles route, the vehicle will decelerate and stop.
Vehicles will decelerate when approaching the position of a
Acceleration / Deceleration constraints#
Vehicles can only change their speed by a certain amount each time step.
This is defined by the
departSpeed / arrivalSpeed#
Variable Speed Signs#
Variable speed signs are used to modify the speed limit of an edge for a defined time interval.
Calibrators are used to adapt the flow on an edge for a defined time interval but may also be used to modify the speed limit of an edge.
Vehicle devices are a way to customize vehicle behavior or create additional output. The following devices can impact vehicle speed:
- glosa : slow down and speed up to smooth speed near traffic lights
- driverstate : random changes to speed based on modelled perception errors with regard to car-following gap and speed difference
Vehicles can forced to adapt their speed using TraCI commands. When using the command slow down stochastic influences on speed are not applied. By using the speed mode command, various safety related influences can be selectively disabled.