VISSIM 5.30-05 User Manual

6 The Traffic Network
routing decision!) are affected by the second direction decision and thus the
percentage of left-turn vehicles will be reduced by 20%.
A direction decision becomes effective the time step after the vehicle has
passed the decision point; thus the distance between decision point and
the next following connector should be sufficient. The minimum distance
depends on the speed of the fastest vehicles. If the fastest vehicle is
traveling with a speed of 20 meters per second, the minimum distance
between direction decision point and connector is 20 meters if the model is
running at one time step per simulation second.
On a multiple lane section vehicles with an assigned direction decision ‘left’
will use the leftmost lane while vehicles with an assigned direction decision
‘right’ will move to the far right lane
A direction decision can affect all vehicles that have not been assigned to a
route. If a vehicle with an assigned direction decision passes another
direction decision point and gets a new direction decision assigned to, then
the new decision overwrites the previous one (see example above).
A vehicle’s direction decision is only reset when passing a connector with
the appropriate direction setting. Otherwise it keeps the direction decision
until it leaves the network, gets a new direction assignment or passes a
direction decision with the ‘none’ criteria.
Setting the desired direction to “none” resets the direction decision for
vehicles affected by that decision point.
6.4.7 Routing Decisions versus Direction Decisions
Routes do have several advantages compared to direction decisions. Thus it
is recommended always to use routing decisions.
► While direction decisions only affect a single lane, routes capture traffic
on all lanes, thus reducing coding effort.
► Routes do not require the cumbersome calculation of turning
percentages when the traffic flow is distributed between more than two
directions.
► Modeling traffic flow with routing decisions guaranties an accurate
replication of merge situations. In contrast to turning decisions, routing
decisions force vehicles to follow predefined link sequences, even if that
means waiting at a connector for a gap to merge. Vehicles that have
been assigned a direction decision would just continue if they cannot find
a gap to merge, requiring the use of special “tricks” for realistic modeling.
► Routes allow for accurate modeling of traffic flow through multiple
intersections and turning decisions. This is in contrast to modeling with
turning decisions where the origin of vehicles is forgotten after each
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