*Article* **Solar-Powered Active Road Studs and Highway Infrastructure: Effect on Vehicle Speeds**

**Richard Llewellyn \*, Jonathan Cowie and Grigorios Fountas**

Transport Research Institute, Edinburgh Napier University, Edinburgh EH10 5DT, UK; j.cowie@napier.ac.uk (J.C.); g.fountas@napier.ac.uk (G.F.)

**\*** Correspondence: r.llewellyn@napier.ac.uk; Tel.: +44-(0)131-455-3755

**Abstract:** Vehicle speeds have a direct relationship with the severity of road crashes and may influence their probability of occurrence. Solar-powered active road studs have been shown to have a positive effect on driver confidence, but their impact on vehicle speed in conjunction with other road features is little understood. This study aims to address this gap in knowledge through a case study of a 20 km section of a strategic major road featuring a variety of highway infrastructure features. Before-and-after surveys were undertaken at 21 locations along the route using manual radar speed measurement. Analysis of nearly 10,000 speed measurements showed no statistically significant change in mean speeds following the implementation of the road studs. Linear regression models are proposed for two different posted speed limits, associating road features with expected vehicle speed. The models suggest that vehicle speeds are chiefly influenced by merges, curves, gradients, and ambient light conditions. The findings of this study should provide confidence that active road studs may be implemented without a negative impact on speed-related safety. The work also provides further expansion of the evidence base describing the effect of highway infrastructure features on vehicle speeds.

**Keywords:** active road studs; highway infrastructure; vehicle speeds; road safety; road features
