The impact of precipitation on speed-flow relationships along a UK motorway corridor

Although the fundamental traffic diagram provides the characteristics of a typical road traffic speed-flow relationship, little consideration has been given to the impact of adverse weather conditions on the relationship and the subsequent impact on local speed-flow. For the first time, this study uses precipitation radar along with a state-of-the art traffic information system to ascertain the relationship between speed-flow and precipitation on a UK transport corridor at the local (junction to junction) scale. It is evident that precipitation causes a significant reduction in speed and maximum flow on many links of the corridor as well as a downward reduction in the overall speed-flow relationship. With increased instances of heavy precipitation predicted in the UK as a result of climate change, these findings highlight the subsequent impact on journey travel times and associated economic costs.