Pavement skid resistance consideration in rain-related wet-weather speed limits determination

Posted maximum speed limits used by highway agencies worldwide are typically applicable to dry-weather driving conditions. During wet weather, these speed limits may exceed the maximum speed for safe traffic operations. More and more highway agencies are employing variable speed limits as a means to improve wet-weather driving safety. For rain-related wet-weather conditions, the variable speed limit systems in use today commonly set speed limits based on either consideration of stopping sight distance, or past experience and engineering judgement. A common shortcoming of the existing systems is that the actual pavement skid resistance characteristics are not considered in setting the speed limits. This paper proposes an improved procedure to derive mechanistically the speed limits from the required safe stopping distance as well as the minimum skid resistance requirement for safe driving, based on the actual skid resistance properties of the pavement considered. The proposed procedure makes use of a finite-element skid resistance computer simulation model to determine both the available skid resistance values and vehicle braking distances at different vehicle speeds of the in-service pavement under specified rain-related wet-weather conditions. The procedure is theoretically sound, and is consistent with the concept of pavement network friction management system. In the analysis, the rain-related wet-weather condition is represented by rainfall intensity, and the road section is characterised by its gradient and cross slope, as well as the skid resistance properties of the pavement. A numerical example is presented to illustrate the working of the proposed procedure.