Changing the desired speed

In a Vissim network, you can define the desired speed of vehicles based on curve radius.The speed is then either controlled automatically by Vissim or through network objects of the types Reduced speed areas and Desired speed decisions at relevant positions within the Vissim network. Whereas with automatic speed limitation in curves the vehicle reacts to one link intermediate point with radius at a time and starts braking in good time if necessary, reduced speed areas have an effect on the entire area for which they are modeled.

• Automatic reduction of the speed of forward driving vehicles in curves is selected by default in the Network settings (Selecting network settings for vehicle behavior). With this setting, you do not have to adjust the desired speed of vehicles after making changes to the link geometry. This generally becomes necessary when you use Reduced speed areas and Desired speed decisions. The automatic reduction of the speed does not cause any delay.

However, for some use cases, it may be useful to define reduced speed areas or desired speed decisions to control speed specifically:

• Reduced speed areas for a temporary change in speed over a specific link length. Example: Multitraction public transport vehicles should continue to travel slowly after exiting a curve so that the last of the multitraction vehicles also takes the curve slowly (Using reduced speed areas to modify desired speed).
• DesSpeedDecisions (desired speed decisions) for a permanent change, for example on motorways, for traffic signs which limit the speed, or at the entry and exit of a town. DesSpeedDecisions are placed at the position where the change is to begin. For multi-lane links, position a desired speed decision on each lane and define the attributes (Using desired speed to modify desired speed decisions).

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. This is when its desired acceleration is used.

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.