Lec 12 TD-Part 5: ch5.4.4, H/O, pp.498, Intro to trip assignment Know the purpose of trip assignment Know a few names of trip-assignment procedures Know the difference of minimum-path techniques and minimum-path with capacity restraints Lecture Objectives

Trip Assignment Trip assignment is the procedure by which the planner predicts the paths the trips will take. The planner can get realistic estimates of the effects of policies and programs on travel demand.

Minimum-path techniques (+ “all-or- nothing” trip loading) Assumption: Travelers want to use the minimum impedance route between two points. Given impedance values, assignment algorithms find minimum paths (or shortest paths) to get from point A to all other locations (to which trips are distributed). A skimtree is created from 1 to all other nodes (see next slide).

Minimum-path techniques (+ “all-or- nothing” trip loading) (cont) Travel timePrevious node A skimtree is created from 1 to all other nodes. In this method, all trips between a given origin and destination are loaded on the links comprising the minimum path and nothing is loaded on the other links.  No consideration of the link capacities.

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)