Drivers theoretically acknowledge the need to reduce their speed in inclement weather, but in practice only alter their speed marginally. This paper examines traffic behaviour in three weather categories: fine, rain and misty conditions. Weekly spot speed surveys undertaken on the M4 motorway, south Wales are analysed. Each survey recorded the speed (mph) of 200 vehicles in the outside lane of the eastbound, dual 2-lane carriageway. Manual observations were made also concerning the weather at the time of each survey. For consistency commuter speeds occurring on the same weekday peak hour were recorded for a six month period (8-9 am Tuesday mornings), thus, effectively controlling many other external factors that might influence vehicle speeds. Hourly traffic flow information was obtained from an automatic traffic counter located at the survey site. Analysis of this not only confirms the peak hour, but also the consistency of the commuters' daily travel routine. Speeds in inclement weather are compared with those in fine conditions (the control). This study finds a small, but significant reduction in mean speeds in both wet weather and misty conditions, and yet such speed modifications are not sufficient to compensate for the increase hazard posed by inclement weather. (Author/publisher).
Abstract