Analysis of Probability of Missing an Overhead Sign due to Heavy-Vehicle-Induced Occlusion: Application to Sign Design

Traffic signs are an important means of communication with drivers on road. With the increase in multi-lane expressways the use of overhead signs has also increased. Like with sideway signs a driver can also miss an overhead sign due to occlusion of the sign by a leading heavy vehicle. Little or no research has been carried out to mathematically analyse conditions under which the driver of a light vehicle will not be able to read a sign due to a leading heavy vehicle. The lack of an analytical understanding of this phenomenon is reflected in the available design standards of the day. This paper analyses the conditions that lead to occlusion of overhead signs because of a leading heavy vehicle. Next, the duration for which the occlusion occurs is determined. Subsequently the results on duration of occlusion are used to evaluate whether enough time is left for the driver to read the sign. If enough time is not available then the driver is said to have “missed” the sign. The analysis indicates that there exists regions in the speed-density (u-k) space where a driver in the light vehicle will miss the sign. Assuming that the speeds of the light and heavy vehicles and the distance headway between them vary randomly, the probability of missing a sign is also obtained. This is used to determine a sign design (in terms of sign features and repetitions of the sign) which keeps the probability of missing the information on a sign below a threshold.