How Additional Road Capacity Affects Travel Demand
In May this year, the DfT published a ‘think-piece’ by Professor of Transport Policy Phil Goodwin on How Additional Road Capacity Affects Travel Demand.
It is quite a technical report and rather than going into all the detail in it, I have relied mainly on Professor Goodwin’s Summary.
First though there is his Preface, which sums up his conclusion quite nicely.
‘In a sentence, I confirm that the core problem established by SACTRA over 30 years ago still exists: building extra road capacity to solve congestion from increasing traffic is likely to induce extra road traffic, which receives benefits itself but causes additional delays and environmental costs to others’.
One early example of induced demand is in a 1938 Ministry of Transport study of the 1925 opening of the Great West Road. It said:
‘As soon as opened, it carried 41-42 times more vehicles than the old route was carrying: no diminution, however, occurred in the flow of traffic on the old route, and from that day to this, the number of vehicles on both roads has steadily increased.
These figures serve to exemplify the remarkable manner in which new roads create new traffic’.
Some of the mechanisms for induced demand include:
- rescheduling of vehicle trips to take advantage of changed peak period conditions
- increasing the frequency of vehicle trips between a given origin and destination
- switching from cycling, walking or public transport to private vehicle for existing trips
- decreases in vehicle occupancy for personal travel
- shopping at more distant destinations
- changes in patterns of land use
Summary
The first paragraph of the Professor’s Summary encapsulates induced demand nicely.
‘Transport demand varies in response to many different external influences. Where these are expected to cause road traffic growth, there are often plans to provide extra road capacity to cope with it.
For a period this extra capacity will reduce costs, in time and money, which will itself trigger more traffic growth, called ‘induced traffic’.
The travel which makes up this induced traffic is there because it receives benefits. There is generally the potential for similar induced demand for all modes and contexts.
The process also applies in reverse, when road capacity is reduced, or reallocated to public transport, active travel or other functions’.
He goes on to say that induced traffic can cause additional costs to other road users, and non-road users, particularly congestion, environmental and social costs.
Induced demand matters because it affects forecasts, appraisal, and retrospective evaluation of projects.
If no account is taken of induced traffic, the tendency is to over-estimate how bad conditions will be without the project, and how good they will be with the project.
National Traffic Forecasts from 1989 to 2025 have systematically over-estimated traffic growth.
From 2002 to 2024, National Highways carried out Post-Opening Project Evaluations (POPEs), but they used mistaken methods and under-estimated induced traffic.
In principle, National Highways adjusted the evaluation method in 2024, but changes to their POPE reports have only started to be seen this year.
Econometric modelling can distinguish between travel demand due to external trends, and that resulting from specific interventions.
On reduction of road capacity, Professor Goodwin says:
‘…where road capacity had been reduced or reallocated as an act of policy, and some accidents, to achieve intended benefits to public transport, walking, cycling, quality and economic efficiency of town centres…the corresponding reduced traffic effect of the induced traffic [was found]: just as the benefits of induced traffic from increased road capacity were often less beneficial than intended, so the disbenefits of reduced traffic from reductions in road capacity were often less than feared’.