Passing Sight Distance and Crest Vertical Curve Design In addition to SSD, in some instances, it may be desirable to provide adequate passing sight distance, which can be an important issue in two-lane highway design (one lane in each direction). Passing sight distance is a factor only in crest vertical curve design because, for sag curves, the sight distance is unobstructed looking up or down the grade, and at night, the headlights of oncoming or opposing vehicles will be seen. In determining the sight distance required to pass on a crest vertical curve, Eqs. 3.13 and 3.14 will apply; however, whereas the driver’s eye height, H1, will remain at 3.5 ft, H2 will now also be set to 3.5 ft. This value for H2 is the assumed value for the portion of a vehicle’s height necessary to be visible such that it can be recognized as an opposing vehicle to a driver performing a passing maneuver. Using the same height for both H1 and H2 provides a reciprocal design relationship; that is, if the driver of the passing vehicle can see the opposing vehicle, then the opposing
vehicle driver can see the passing vehicle. Substituting these H values into Eqs. 3.13 and 3.14 and letting the sight distance S equal the passing sight distance, PSD, gives
For PSD < L
For PSD > L
= × − 2800 2 PSDmL
where Lm = minimum length of vertical curve in ft,
A = absolute value of the difference in grades (|G1−G2|), expressed as a percentage, and
PSD = passing sight distance in ft.
As was the case for SSD, it is typically assumed that the length of curve is greater than the required sight distance (in this case, L > PSD), so
= 2PSD 2800 K (3.26)
K = horizontal distance, in ft, required to effect a 1% change in the slope (as in Eq. 3.10), and
PSD = passing sight distance in ft.
The passing sight distance (PSD) used for design is assumed to consist of four distances: (1) the initial maneuver distance (which includes the driver’s perception/reaction time and the time it takes to bring the vehicle from its trailing speed to the point of encroachment on the left lane), (2) the distance that the passing vehicle traverses while occupying the left lane, (3) the clearance length between the passing and opposing vehicles at the end of the passing maneuver, and (4) the distance traversed by the opposing vehicle during two-thirds of the time the passing vehicle occupies the left lane. The determination of these distances is undertaken using assumptions regarding the time of the initial maneuver, average vehicle acceleration, and the speeds of passing, passed, and opposing vehicles. The sum of these four distances gives the required passing sight distance. The reader is referred to for a complete description of the assumptions made in determining required passing sight distances.
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