E 443 Passing Sight Distance For Two-Lane Highways

On two-lane highways, the overtaking of slower moving vehicles must be accomplished on a lane regularly used by opposing traffic. Therefore, the driver of the overtaking vehicle must see far enough ahead to permit enough time for a safe completion of the passing maneuver. Sufficient distance must be available to enable the passing vehicle to return to the right lane without cutting off the slower vehicle and before meeting the opposing traffic.

The minimum passing sight distance is based on the following assumptions:

1. A single vehicle is passing a single vehicle.
2. The overtaken vehicle travels at a uniform speed.
3. While the passing vehicle is in the opposing lane its average speed will be 10 miles per hour faster than the overtaken vehicle.
4. There is adequate distance between the oncoming vehicle in the opposing lane and the overtaken vehicle at the time the passing vehicle returns to its own lane.

The minimum length of passing sight distance for a two-lane highway is determined by the sum of the four distances d₁ + d₂ + d₃ + d₄, as shown in Figure E 343. The distance-time relationships presented below are based on field observations of driver behavior during passing maneuvers where:

d₁ = the distance traveled during perception and reaction time and during the initial acceleration to the point of encroachment on the left lane.

d₂ = the distance traveled while the passing vehicle occupies the left lane.

d₃ = the distance between the passing vehicle at the end of its maneuver and the opposing vehicle.

d₄ = the distance traversed by an opposing vehicle for two-thirds of the time the passing vehicle occupies the left lane, or two-thirds of d₂.  For additional information, refer to Section E 020F(1b).

E 443.1 Passing Sight Distance on Crest Vertical Curves

In determining the required passing sight distance on a crest vertical curve, it is assumed that an object being passed on the highway has a height above the pavement of four feet.

Figure E 343.1 shows a series of curves for determining minimum passing sight distance where the sight distance is either greater or less than the length of curve. Formulas for determining this distance are also included in the figure.

Generally, it is impractical to design crest vertical curves to provide for passing sight distance because of the high cost where cuts are involved and the difficulty of fitting the required long vertical curves to the terrain of existing improvements for high-speed roads.

There are no widely used criteria for passing sight distance on sag vertical curves. The controls previously discussed are:

1. Headlight sight distance.
2. Rider comfort.
3. Drainage control.
4. Pleasing appearance.

These are all taken into consideration, with the headlight sight distance control usually prevailing.

E 443.2 Passing Sight Distance on Horizontal Curves

There are few projects in the City in which provisions must be made for passing sight distance on horizontal curves.  This situation may occur in the more rural areas, where prevailing traffic density is low and intersections are widely spaced.  Where this type of project is encountered, the minimum passing sight distance that must be provided for a two-lane-highway is about four times greater than the stopping sight distance for the same design speed. It follows that the clear distance D between the curb line and the obstruction for normal highway cross-sections in cut should be from four to twelve feet greater than those previously presented for stopping sight distance.