Editing Route assignment (section)

===Long-standing techniques===

The problem of estimating how many users are on each route is long standing.  [[Urban planner|Planners]] started looking hard at it as [[Controlled-access highway|freeways]] and expressways began to be developed.  The freeway offered a superior level of service over the local street system, and diverted traffic from the local system.  At first, diversion was the technique.   Ratios of travel time were used, tempered by considerations of costs, comfort, and [[Level of service (transportation)|level of service]].

The [[Chicago Area Transportation Study]] (CATS) researchers developed diversion curves for freeways versus local streets.  There was much work in California also, for California had early experiences with freeway planning.  In addition to work of a diversion sort, the CATS attacked some technical problems that arise when one works with complex networks.  One result was the [[Bellman–Ford–Moore algorithm]] for finding [[shortest path]]s on networks.

The issue the diversion approach did not handle was the feedback from the quantity of traffic on links and routes.  If a lot of vehicles try to use a facility, the facility becomes [[Traffic congestion|congested]] and travel time increases.  Absent some way to consider feedback, early planning studies (actually, most in the period 1960-1975) ignored feedback.  They used the Moore algorithm to determine [[Shortest path problem|shortest paths]] and assigned all traffic to shortest paths.  That is called [[all or nothing assignment]] because either all of the traffic from ''i'' to ''j'' moves along a route or it does not.

The all-or-nothing or shortest path assignment is not trivial from a technical-computational view.  Each traffic zone is connected to ''n - 1'' zones, so there are numerous paths to be considered.  In addition, we are ultimately interested in traffic on links.  A link may be a part of several paths, and traffic along paths has to be summed link by link.

An argument can be made favoring the all-or-nothing approach. It goes this way: The planning study is to support investments so that a good level of service is available on all links. Using the travel times associated with the planned level of service, calculations indicate how traffic will flow once improvements are in place. Knowing the quantities of traffic on links, the capacity to be supplied to meet the desired level of service can be calculated.