Why do the MUTCD and the AASHTO Green Book have different values for passing sight distance?
Like many aspects of highway design, the values used for design of a new facility are often more conservative than what operating agencies find to be “reasonable minimums” for actual operating highways, given the practicalities involved and the wide mix of vehicle types, driver behaviors, etc. NCHRP Report 605 should be consulted for a full discussion of why the different values have existed. That report concluded that the AASHTO Green Book should be modified to use passing sight distance values that are the same as those in the MUTCD.
