Effect of Roadway Geometrics and Large Trucks on Variable Message Sign Readability

One of the most critical aspects of a variable message sign (VMS) is that the sign provide a legibility distance that meets or exceeds the required reading distance by a driver. Whereas the VMS design itself dictates the maximum legibility distance that is possible from a given sign, the distance at which it can be truly seen by motorists is dependent upon a number of external factors. In this paper, researchers present mathematical models to evaluate the influences of the following three types of VMS viewing constraints: Crest vertical curves, Horizontal curves where roadside obstructions may be present, and Roadways carrying a significant number of large trucks. The models described and examples shown illustrate that vertical curves, horizontal curves (where roadside obstructions exist), and large trucks can each limit the distance at which a VMS can be read, depending on the characteristics of those factors. This in turn translates into shorter available reading times and the need to display a shorter message with fewer units of information on the VMS. The paper should be beneficial to both traffic control system designers who select VMS placement and installation characteristics (mounting height, orientation, etc.) and to VMS operators who design and select the messages placed on the signs themselves.