4.11.3 Passing Sight Distance

Passing sight distance is the distance required by a driver to make a passing maneuver without cutting off the passed vehicle and before meeting an opposing vehicle. Therefore, passing sight distance is applicable to two-lane highways only (including two-way frontage roads).
Recommended passing sight distances are based on the following assumptions:
  • The speeds of the passing and opposing vehicles are equal and represent the design speed of the highway;
  • The passed vehicle travels at a uniform speed and the speed difference between the passing and passed vehicles is 12 mph;
  • The passing vehicle has sufficient acceleration capability to reach the specified speed difference relative to the passed vehicle by the time it reaches the critical position, which generally occurs about 40 percent of the way through the passing maneuver;
  • The lengths of the passing and passed vehicles are 19-ft;
  • The passing driver’s perception-reaction time in deciding to abort passing a vehicle is 1-s;
  • If the passing maneuver is aborted, the passing vehicles will use a deceleration rate of 11.2 ft/s
    2
    , the same deceleration rate used in stopping sight distance design criteria;
  • For a completed or aborted pass, the space headway between passing and passed vehicles is 1-s; and
  • The minimum clearance between passing and opposing vehicles at the point at which the passing vehicle returns to its normal lane is 1-s.
In the design of two-lane highways, minimum or greater passing sight distance should be provided wherever practical, since less than minimum distances reduce the safety and LOS of the roadway. For rolling terrain, provision of climbing lanes may be a more economical alternative than achieving a vertical alignment with adequate passing sight distance.
The minimum passing sight distance for a two-lane road is about twice the minimum stopping sight distance at the same design speed. To meet those greater sight distances, clear sight areas on the inside of curves should be provided. For cut sections, designing for passing sight distance should be limited to tangents and very flat curves. Even in level terrain, providing passing sight distance would need a clear area inside each curve that would, in some instances, extend beyond the normal ROW line.
Minimum passing sight distance values for design of two-lane highways are shown in
Table 4-26
.
These distances are for design purposes only and should not be confused with other distances used as warrants for striping no-passing zones as shown in the
TMUTCD
. For the design of typical two-lane rural highways, except for level terrain, provision of near continuous passing sight distance is impractical. However, the designer should attempt to increase the length and frequency of passing sections where economically feasible.
Table 4-26: Passing Sight Distance
K-Values for Determining Length of Crest Vertical Curve for Various Passing
Sight Distances (PSD)
K-Values for Determining Length of Crest Vertical Curve for
Various Passing Sight Distances (PSD)
Design Speed (mph)
Minimum PSD for Design (ft.)
Minimum K-Value
1
20
400
57
25
450
72
30
500
89
35
550
108
40
600
129
45
700
175
50
800
229
55
900
289
60
1000
357
65
1100
432
70
1200
514
75
1300
604
80
1400
700
Notes:
  1. Based on eye height = 3.5 ft and object height = 3.5 ft; K = PSD2 / 2800.