VISSIM 5.30-05 User Manual

6.3.3 Desired Speed Changes
Network Coding
Whenever there is a change of free flow speed in the VISSIM network, a
speed distribution change is to be defined. There are two ways of defining
speed distribution changes:
► Reduced Speed Areas (Temporary change)
► Desired Speed Decisions (Permanent change)
Speed changes are required for modeling the following:
► bends, curves and turning lanes at intersections,
► any speed limits,
► bottlenecks.
Both of them are modeled as network elements in VISSIM. The main
difference between the two is, that with reduced speed areas a (faster)
vehicle automatically decelerates prior to the start of the reduced speed
area to get the speed defined for its vehcile class for that reduced speed
area right at the start of it. After passing the reduced speed area the vehicle
automatically accelerates to the desired speed that previously was assigned
to it. In contrast, a desired speed decision does not affect the vehicle before,
but when passing the decision cross section.
Each vehicle gets a fixed fractile value for speed distributions assigned when
entering the network. For example, if the fractile is 40%, the vehicle will
always get the 40% percentile of the desired speed distribution at desired
speed changes. If the fractile is 100%, the vehicle will always get the
maximum speed value of the distribution.
Reduced speed areas and desired speed decisions can also be labeled
with the numbers of the assigned speed distributions. If there is only one
distribution, the lower and upper limit of the distribution is displayed
instead. Via VIEW - NETWORK ELEMENTS..., the label display can be switched
on or off.
6.3.3.1 Reduced Speed Areas
When modeling short sections of different speed characteristics (e.g.
curves or bends), the use of reduced speed areas is advantageous over the
use of desired speed decisions.
Reduced speed areas are typically used for curves (e.g. turning movements).
Thus they are normally placed on connectors rather than links.
For multi-lane links reduced speed areas need to be defined for each lane
separately. Thus different characteristics can be defined for each lane.
A reduced speed area cannot reach across multiple links. However, multiple
consecutive areas (one for each link) can be created and placed on
consecutive links. If two reduced speed areas with the same properties are
placed close to each other then the vehicles affected by them will continue
with the new speed even between the two areas.
User Manual © PTV AG 2011 201