Methods in use to forecast and assign traffic in planning of major highway facilities are reviewed. Traffic assignments were computed based on both travel time and distance parameters using various diversion curves. Network traffic assignment methods were reviewed with regard to highway capacity restraint functions. These results were compared with the actual travel data. An analysis is presented of errors related to origin-destination input with regard to individual links of the network. Further analysis was conducted to relate link assignments to changes in the origin-destination patterns and to network changes. A pair of origin-destination studies, made before and after the construction of a new Connecticut river crossing, were used to test assignments to an isolated rural highway facility. The four techniques tested were the AASHO diversion curve, the California diversion curve, the all-or-none method, and a difference-ratio method. Comparisons of assignment results to 1956 survey results for the two existing bridges indicated that all of the methods used duplicated survey results and that time-based assignments were slightly better than distance-based results. A new assignment technique, in which travel times via all three river crossings were considered simultaneously, duplicated the survey volumes on all three bridges. Traffic assignment to individual links of a network are emphasized so that the resulting volumes can be used by highway and traffic engineers in their geometric design computation. Previous observations were substantiated that travel time is a better parameter than travel distance.
Abstract