The number of lanes is an element of the cross-section of a road. It is determined by expected traffic volumes, traffic composition and design speed. Conversions to a higher number of lanes have often been undertaken to manage higher traffic volumes and improve the traffic flow. On theoretical grounds safety benefits can be expected. Multiple-lane roads provide continuous opportunities for safe overtaking and can therefore have a positive effect on traffic operations by decreasing the interactions between faster and slower vehicles. However, increasing the number of lanes may also increase same-direction conflicts and can lead to an increase in driving speeds. It also increases the crossing distance for crossing traffic (including pedestrians and cyclists). The effects of changing the number of lanes on road safety depends on a number of closely interrelated factors, including Average Daily Traffic (ADT), paved width, roadway type, and cross-section elements.
For both urban and rural roads, conversions of undivided two-lane roads to divided four-lane roads have been found to result in substantial accident reductions. But these large reductions are due to both the change in number of lanes and the physical separation of opposite lanes. The evidence concerning the conversion of 2- lane to 4-lane undivided rural roads is mixed. Depending upon ADT, results may be more or less positive. On high volume roads the conversion is associated with safety benefits, but not on lower volume roads. The evidence concerning the conversion of 4-lane or 5-lane roads to 5 or 6- lane roads indicates that road safety is not improved or may be negatively affected.
