Lec 13 TD-Part 6: Minimum-path with capacity restraints and trip assignment example problems Example 14: Another traffic assignment example Know the difference of minimum-path techniques and minimum-path with capacity restraints Example 13

Minimum path with capacity restraints Travel time increases as traffic volume on the link increases because of interaction between the drivers and their perception of safety because they slow down as volume increases. Capacity restraints attempts to balance the assigned volume, the capacity, and the related speed (translated to travel time). Several methods are available, but the most popular one is the Bureau of Public Roads model. Once you reach this point travel time exponentially increases.

The BPR model Where TQ = travel time at traffic flow Q T0 = “zero-flow” or “free-flow” travel time = travel time at practical capacity x 0.87 Q = traffic flow (veh/hr) Qmax = practical capacity = ¾ x saturation flow alpha & beta = parameters (need calibration) See the example on page 500. Note that this example assumes that the ratio of daily flow/capacity is equal to hourly flow/capacity.

Two other methods discussed in the text Davidson’s model Tau = LOS parameter, for freeways, for urban arterials, for collector roads Greenshields’ model (See Example 13)