Use of a 2 1/2 foot high target in setting the end of a no-passing zone is a safety factor.It is based on an assumed height of vehicle headlights of 2 1/2 feet. The stipulation that the 2 1/2 foot target must be seen throughout a dip in a vertical curve is simply an additional safety factor. The need for these safety factors cannot be readily proved or disproved on paper. A check on the policies of 17 other states throughout the United States reveals that Michigan is unique in use of the 2 1/2 foot target height (although some states go much lower, they combine the lower height with a much shorter sight distance). The majority of states checked conform to the national standard (15 which is based on the following premise: (16 While the headlights of a vehicle are only about 2 feet above the pavement, a reduction kin the value of the assumed height of object is not necessary. In the case of sight distance for safe passing at night, the beams of the headlights of an opposising vehicle generally are seen sometime before the headlights come into view and even before the top of a vehicle can be seen at the same location in the daytime. Apparently, as with sight distance, the answer to this question of need for a lower object height lies in what its use produces in the field.
Abstract