By Derek T. Jander, P.E.
Drivers on a road often think of safety and accident prevention in terms of speed limits, STOP signs, and other visible roadway characteristics that send the clear message to, among other things: Don’t drive so fast! Slow down! Stop ahead! However, the civil engineering design of a roadway also includes the consideration of sight distance. For the safety of motorists and pedestrians, a civil engineer evaluates the ability of a motorist to observe a condition ahead that requires attention, recognize the need to slow or stop, to apply the brakes and bring the vehicle to a safe, controlled stop. And of course, this is directly related to the posted speed limit.
What is sight distance and how is it calculated?
According to the American Association of State Highway and Transportation Officials’ (AASHTO) A Policy on Geometric Design of Highways and Streets (aka the Green Book), sight distance is noted as needed for “A driver’s ability to see ahead… for safe and efficient operation of a vehicle on a highway.” There are four (4) primary types of sight distance the AASHTO Green Book addresses:
There are different factors that influence the determination of sight distance for each of the four (4) primary sight distance types. Specifically, the height of a driver’s eye and the height of an object and/or obstacle on the roadway in front of a driver directly influence the calculations and/or studies that determine sight distance.
For civil engineering design, sight distance considers the height of a driver’s eye in a passenger vehicle to be 3.5 feet above the road surface. Note that this is for passenger vehicles. A driver in other vehicles may have lesser or greater available sight distance; however, unless special circumstances are present, the 3.5 feet for passenger vehicles is the typical value used for sight distance calculations.
Evaluating sight distance is not necessarily that simple, though. The four (4) primary types of sight distances have varying considerations for the determination of sight distance. The types of sight distance considerations are as follows:
As one can see, the dynamics and perspectives involved with understanding sight distances are variable and unique to every situation. When investigating an incident on a roadway or intersection, the sight distance may be a factor to consider in assessing the causes for a roadway accident. Some examples of some things to consider:
In any vehicle accident, the circumstances and location are always unique. However, it is important to understand that data can be captured, either through measurements or more technology-oriented scanning techniques, that can document the incident scene to help uncover valuable facts and/or data in your case. Your expert can help you with obtaining that important data.
For all drivers out there, understand this about roadway design: a civil engineer has considered a person’s perception-reaction time and a vehicle stopping distance to determine the necessary sight distance along a given length of roadway. But no matter the available sight distance, speeding causes accidents. For every 10 mph over the posted speed limit, the vehicle is traveling nearly 15 feet more per second, and the vehicle stopping distance is longer. In addition, there is an assumption that road worthy vehicles have maintained braking and suspension systems to bring a vehicle to a stop within expected distances. And not to overlook distracted driving, taking your eyes off the road for only one second while driving 60 mph, your vehicle has traveled 88 feet without attention, which may be 88 feet of lost stopping distance and extra energy in a collision. So, for the safety of you and your loved ones, follow the posted speed limit, maintain your vehicle, and stay safe. And please, pull over and stop when using your phone.
Feel free to contact us to speak with Mr. Jander or one of our other experts in regard to accidents involving vehicles and/or pedestrians on roadways, parking lots, sidewalks, or other areas.
Contact Mr. Jander: firstname.lastname@example.org / 610-889-0771 (P)