Using desired speed to modify desired speed decisions

You can permanently change desired speeds with desired speed decisions, for example on motorways, for traffic signs which limit the speed, or at the entry and exit of a town. Desired speed decisions are placed at the position in the network where the change is to begin. For multi-lane links, position a desired speed decision on each lane and define its attributes.

With desired speed decisions, a vehicle only reduces its speed once it has entered a desired speed decisions section. The vehicle is then assigned a new desired speed and changes its speed accordingly. To do so, it uses the desired acceleration or desired deceleration.

When entering the network, each vehicle is assigned a fractile value for speed distribution. This value remains unchanged during the entire simulation time. If this value is 40 %, the vehicle is always assigned a 40 percentile of the desired speed distribution for desired speed decisions. If the fractile is 100 %, the vehicle is always assigned the highest distributed speed.

Calculation of delay measurement for desired speed decisions

After the vehicle has traversed the desired speed decision, Vissim calculates a theoretical speed over several time steps until this speed is identical with the new desired speed of the vehicle. This theoretical speed starts at the original desired speed and ends at the new desired speed. During the time steps the vehicle needs to reach its new, desired speed, Vissim compares the current speed of the vehicle with the current theoretical speed calculated - not with the new desired speed the vehicle is supposed to reach - in order to create a basis for delay measurement calculation.

Notes: You define speed limits over a short distance with reduced speed areas, e.g. for turns at intersections (Using reduced speed areas to modify desired speed). If you use a desired speed decision for this, you must define a second desired speed decision immediately after this, so that the vehicles are given their original desired speed. Vehicles take into account automatic speed limits in curves if this option is selected in the network settings (Selecting network settings for vehicle behavior).