The Impact of Road Investment

Assessing the Impact of Road Investment on Traffic and the Environment in Great Britain

Researcher: Dr Rosa Sanchis-Guarner

Funder: British Academy

Duration: September 2015 – September 2018

Tackling traffic congestion is crucial for British roads as around 90% of passenger and 70% of freight transportation takes place by road. This project assessed the impact of road interventions on congestion and pollution in Britain to establish if there is a causal link between changes in road capacity and traffic flows, and to investigate what drives changes in traffic. This research informed policy makers about the effectiveness of road investments in relieving traffic congestion.

This research resulted in the construction of a unique dataset of regional road projects, traffic flows, emissions and economic outcomes from 1998-2013, leading to the development of a methodology for improving current approaches to traffic flow studies. The fundamental law of road congestion (Downs 1962) suggests that increasing road capacity will only lead to a greater number of journeys being made and thus levels of traffic congestion will remain consistent. Using new data, this project tested the applicability of this model in the UK context where policy restrictions on tax and planning are more stringent. Finally, it attempted to quantify the costs that traffic imposes on the environment and what climate change mitigation policies can be adopted. Fundamentally, this work explored the causal effects between policy intervention and outcome which is crucial for designing relevant and meaningful policy conclusions.

Key research questions:

  • How has road provision in the last 15 years affected traffic volumes in Britain?
  • What mechanisms explain changes in traffic volumes where road interventions have taken place?
  • How do changes in traffic volume affect the environment?

Research partners

  • Wharton School of Business
  • Imperial College Centre for Transport Studies