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:
- Stopping Sight Distance: Providing enough visible distance ahead on a roadway to permit a vehicle to come to a hurried, full stop.
- Decision Sight Distance: Providing enough visible distance ahead to detect an object or decision point and react to safely maneuver. For example, an exit at a highway interchange or the end of a lane.
- Passing Sight Distance: Providing enough visible distance ahead of oncoming traffic to safely decide to pass and then overtake a slower vehicle.
- Intersection Sight Distance: Providing enough visible distance at intersections to observe approaching vehicles and control the vehicle to avoid a collision.
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:
- Stopping Sight Distance: The height of the object and/or obstacle on the road used for the calculations is 2 feet. According to the AASHTO Green Book, 2 feet is “representative of the height of automobile headlights and taillights.” The AASHTO Green Book also goes on to say, interestingly, “objects with heights less than 2 feet are seldom involved in crashes.” The AASHTO Green Book uses a cost-benefit approach here by mentioning that designing for object heights less than 2 feet would increase construction costs without a documented decrease in the frequency or severity of crashes.
- Decision Sight Distance: Due to the same rationale in the application of Stopping Sight Distance, the height of the object is also 2 feet.
- Passing Sight Distance: The object height is 3.5 feet, which is based on vehicle height and represents the portion of the vehicle height that needs to be visible for another driver to recognize the oncoming obstacle.
- Intersection Sight Distance: The design for intersection sight distance is based on the same object height and same rationale used in design for passing sight distance, or 3.5 feet.
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:
- Were objects on the side of the road, like a tree or concrete barrier, obstructing a view?
- Who had responsibility for building and/or maintaining those obstructions?
- Does a sharp curve, either horizontal or vertical, limit the sight distance?
- Was the speed limit appropriate given the curves or the presence of obstructions?
- Design standards change and are updated over the years, so how does the age of the roadway affect the sight distance assessment?
- Were traffic control devices (signs/signals/lights) appropriately placed to advise drivers where sight distance was limited?
- How might 3-Dimensional High-Definition scanning capture the roadway attributes to be utilized in accident reconstruction analysis, computer animations and computer simulations to pictorially present the factual evidence and data to the jury?
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)