As you drive along our mostly rural two-lane roads, have you ever wondered how the passing and no-passing zones are determined? And, if you are like most drivers, you have likely questioned why pavement markings allow passing where you think it shouldn't, and in other cases, why passing is not allowed where you think it is safe to do so.
To explain how these passing/no passing zones are determined, we go to the Manual on Uniform Traffic Control Devices (MUTCD). In general, on two-way roads where center line markings are installed, no passing zones are established at vertical and horizontal curves where the passing sight distance is less than the minimum permissible distance, depending on the posted speed limit or the speed at which 85 percent of the vehicles are traveling at or below. The greater the speed, the greater the required sight distance in order to establish a passing zone.
Sight distance is measured from a height of 3.5 feet above the road surface, the approximate height a driver's eyes would be in an automobile. From a table of values, the sight distance is given for various 85th-percentile or posted speed limits.
Let's use an example. After a straight section of road, where passing is allowed, a curve lies ahead. The speed limit for this road is 55 mph. From the "Minimum Passing Sight Distances for No-Passing Zone Markings" table in the MUTCD, the minimum passing sight distance for 55 mph is listed at 900 feet. Therefore, the beginning of the no-passing zone is the point at which the sight distance first becomes less than that specified in the table, or in our example, 900 feet.
Objects located on the inside of the curve factor in to the sight distance as well. For instance, if a barn blocks a driver's view around the curve, this would affect the start of the no-passing zone. The no-passing zone would end at a point where the sight distance once again reached 900 feet or more.
The above example was for a curve, but the same engineering is used for a hill, where sight distance is obstructed by the crest of the hill. Again, when the sight distance, as measured 3.5 feet above the pavement, became less than 900 feet, a no-passing zone would begin. It would end at the point where the sight distance again exceeded 900 feet.
These examples were for a speed limit of 55 mph. If the same curve or hill existed in a 30 mph speed zone, the minimum sight distance would only have to be 500 feet for a no-passing zone.
So you can see that passing/no-passing zones are based on engineering studies rather than just estimating where to paint solid yellow lines. And, there is no obligation for you to pass just because the lines say you can. If you are not comfortable in passing, don't. Wait for a better place. No matter how long a no-passing zone is, you will eventually get to a passing zone. Don't take chances.
For more articles on vehicle and traffic law and traffic safety, go to the Traffic Safety Board website at www.franklincony.org/content/Departments/View/24.
Dave Werner can be reached at firstname.lastname@example.org.