An expanded version of the perceptual load model was utilized to systematically explore the influence of four variables on drivers' behavior, namely, perceptual load on the road; perceptual load at the roadsides; initial location of an event (on the road versus at the roadside); and the presence and size of advertising billboards. Eighteen participants took part in two experimental sessions with a driving simulator. One of the sessions included advertising billboards and the other session did not. The results indicated that billboards can have a considerable effect on various aspects of driving, such as the amout of time required to respond to a potentially dangerous event or the number of accidents that occured while driving. An important finding was that the effect of billboards on driving was modulated by the degree of perceptual load.