Most roads and numerous streets are considered to qualify as two-lane, two-way highways on which vehicles frequently overtake slower moving vehicles, the passing of which must be accomplished on lanes regularly used by opposing traffic. If passing is to be accomplished with safety, the driver should be able to see a sufficient distance ahead, clear of traffic, to complete the passing maneuver without cutting off the passed vehicle in advance of meeting an opposing vehicle appearing during the maneuver. When required, a driver can return to the right lane without passing if he or she sees opposing traffic is too close when the maneuver is only partially completed.

Many passings are accomplished without the driver seeing a safe passing  section ahead, but design based on such maneuvers does not have the desired factor of safety. Because many cautious drivers would not attempt to pass under such conditions, design on this basis would reduce the usefulness of the highway. An alternative to providing passing sight distance will be found later in this chapter in the section "Passing Lane Sections on Two-Lane Roads."

Passing sight distance for use in design should be determined on the basis of the length needed to safely complete the normal passing maneuvers. While there may be occasions to consider multiple passings, where two or more vehicles pass or are passed, it is not practical to assume such conditions in developing minimum design criteria. Instead, sight distance is determined for a single vehicle passing a single vehicle. Longer sight distances occur in design and these locations can accommodate an occasional multiple passing.

When computing minimum passing sight distances on two-lane highways for design use, certain assumptions for traffic behavior are necessary, some of which offer a wide choice. The assumed control for driver behavior should be that practiced by a high percentage of drivers, rather than the average driver. Such assumptions follow:

1. The overtaken vehicle travels at uniform speed.
2. The passing vehicle has reduced speed and trails the overtaken vehicle as it enters a passing section.
3. When the passing section is reached, the driver requires a short period of time to perceive the clear passing section and to react to start his or her maneuver.
4. Passing is accomplished under what may be termed a delayed start and hurried return in the face of opposing traffic. The passing vehicle accelerates during the maneuver, and its average speed during the occupancy of the left lane is 15 km/h higher than that of the overtaken vehicle.
5. When the passing vehicle returns to its lane, there is a suitable clearance length between it and an oncoming vehicle in the other lane.

Some drivers accelerate at the beginning of a passing maneuver to an appreciably higher speed and then continue at a uniform speed until the passing is completed. Many drivers accelerate at a fairly high rate until just beyond the vehicle being passed and then complete the maneuver either without further acceleration or at reduced speed. For simplicity, extraordinary maneuvers are ignored and passing distances are developed with the use of observed speeds and times that fit the practices of a high percentage of drivers.

The minimum passing sight distance for two-lane highways is determined as the sum of the four distances:

d1 -- Distance traversed during perception and reaction time and during the initial acceleration to the point of encroachment on the left lane.

d2 -- Distance traveled while the passing vehicle occupies the left lane.

d3 -- Distance between the passing vehicle at the end of its maneuver and the opposing vehicle.

d4 -- Distance traversed by the opposing vehicle for two-thirds of the time the passing vehicle occupies the left lane, or 2/3 of d2 above.