Robert T. Lynch, PE, Senior Collision Reconstruction Engineer ::::

All states have a provision within their respective Vehicle Code pertaining to “following too closely” which states, in general, that “the driver of a motor vehicle shall not follow another vehicle more closely than is reasonable and prudent, having due regard for the speed of the vehicles and the traffic upon and the condition of the highway.”(1) Whenever a rear-end impact occurs, the investigating police officer will typically list “following too closely” as a contributing factor; however, not all drivers that rear-end another vehicle are keeping an unsafe following distance (a.k.a. headway) or are following “more closely than is reasonable and prudent” having due regard for the speed of the vehicles on the highway.

Most states recommend a 3 to 4-second “following distance rule” within their driver’s manual. This rule generally provides for sufficient distance to bring a vehicle to a stop in most driving situations; however, the rule is not conducive for drivers on congested highways where keeping such a distance would allow other vehicles to “cut in line” and effectively reduce safety by increasing the number of potential vehicular conflicts. It is often argued that the following distance rule is rarely observed in practice. Support for this argument is found in the review of the attached Google Earth™ aerial image of I-95 in Philadelphia, PA which illustrates that the majority of drivers within this image are accepting a following distance of 100 feet or less, with an average time headway (assuming the vehicle are traveling at the 55-mph speed limit) of about 1 second. A headway equivalent to 1 second is consistent with published research data on “real world” typical time headways. (2) The acceptance of a reduced headway suggests that drivers are “reading the road” assessing the state of traffic as a whole and not just focusing on the vehicle directly in front of them.