Assessing the Effect of Heavy Vehicles on the Visibility of Traffic Signs at Multilane Highways

Traffic signs are critical for the safe and efficient use of highway facilities. Occlusion of traffic signs by moving or stationary objects renders these important traffic control devices ineffective. Therefore, adequate visibility of traffic signs is an important safety concern on multi-lane highways in general and on limited-access facilities in particular due to the high proportion of heavy vehicles in the traffic stream. This paper presents an investigation into the different factors that affect sign occlusion by heavy vehicles on these highway facilities. Parametric analyses were conducted to explore these effects using a simulation model that was developed exclusively for this research. The variables investigated involved; legibility distance, maximum divergence angle, average speed of passenger cars, relative speed of trucks with respect to passenger cars, lateral offset of traffic sign, number of words in message, traffic level, and heavy vehicles ’ percentage. In general, research results showed that the occlusion of ground-mounted traffic signs by heavy vehicles was very significant for conditions that can be described as typical. Also, results suggest that the factors that have most significant effect on sign occlusion involve legibilit