Car-Following Models#

This overview page collects information regarding car-following models in sumo. It is still a stub and needs be expanded.

Model Parameters#

Most car-following models have their own set of parameters. Because some models have a common ancestry they share some parameters. These are described below.


This parameter is used to decouple the simulation step length (option --step-length) from the frequency of driver decision making. It can be set either in the <vType>-definition as attribute actionStepLength or by setting a default value for all vTypes using option --default.action-step-length <FLOAT>.

By default, the action step length is equal to the simulation step length which works well for the default step length of 1s. When performing sub-second simulation by setting a lower step-length value, it may be useful to maintain a higher action step length in order to model reaction times and also in order to reduce computational demand and thus speed up the simulation.

The action step length works similar to a reaction time (vehicle will not react immediately to changes in their environment) but it also differs from a "true" reaction time because whenever a vehicle has it's action step it reacts to the state in the previous simulation step rather than to the state that was seen in their previous action step. Thus the Perception-Reaction loop is less frequent but still as fast as the simulation step length.


This parameter is intended to model a drivers desired minimum time headway (in seconds). It is used by all models. Drivers attempt to maintain a minimum time gap of tau between the rear bumper of their leader and their own (front-bumper + minGap) to assure the possibility to brake in time when their leader starts braking and they need tau seconds reaction time to start breaking as well.

The use of minGap with respect to the desired (minimum) time headway varies by model. I.e. the Krauss model selects a speed that ensures minGap can always be maintaned whereas other models may not do this.

The real reaction time is fundamentallylimited by the simulation step size (sumo option --step-length <FLOAT>), resp. the action step length (option --default.action-step-length <FLOAT>, parameter ). For that reason a value of tau below this step-size may lead to collisions if the leader suddenly starts braking hard. If tau is larger than step-size the situation is safe but drivers will still react during each simulation step unless a custom action-step-length is used (e.g. by setting option --default.action-step-length).