User Response to New Road Capacity: A Review of Published Evidence

This paper presents a review of the known evidence on the various aspects of user response to new road capacity.

The traffic effects of-new road capacity have important implications for the appraisal of road schemes. The conventional method for inter-urban roads (and increasingly for urban road projects) assumes that the volume of trips, and their destination between pairs of zones, is given. The only response to new investment that is modelled is re-assignment between routes. Relative to this, new road capacity creates the potential for several effects. These effects include:-

(1) Wide area re-assignment, involving re-routing of trips external to the study area.

(2) Redistribution of trips to different destinations.

(3) Attraction of trips from other modes.

(4) Re-timing of trips.

(5) Generation of trips, consisting of trips which are either entirely new or are made more frequently.

Section 2 details the work which has been conducted to analyse the response to particular road construction schemes, this is largely but not exclusively made up of before and after measurements of traffic flows. Section 3 reviews the more diverse work which is not specific to any particular scheme; this work is concentrated on the modal diversion and departure time aspects of user response. Section 4 presents an overview of the literature on land use/ development effects. The final section draws together the available evidence on the scale of each of effects 1-5 above. Of these, trip re-timing is found to be of high importance, especially in the context of urban trips.