The general factors which control the sight distance for highways apply also to sight distances at intersections. See Section E 440, Sight Distance. All approaches to a highway intersection at grade should permit the driver approaching the intersection to have an open view of all points at a sufficient distance to permit control of the vehicle and to avoid collision before reaching an unexpected obstacle. Where signs or signals control traffic at an intersection, the sight distance to the traffic control from an approaching vehicle is sometimes used as a sight distance control. While a right-angle crossing is desired, some deviation is permissible. Angles greater than 60 degrees produce only a small reduction in visibility, while angles smaller than 60 degrees produce a skew- angled intersection with greater reduction in visibility.
At signalized intersections, special provisions need not normally be made for sight distance except that the signals should be visible from adequate distances. However, since burned out bulbs or power failures may make a signal control ineffective at times, and since it is possible that a disabled car or other obstruction may be in the intersection despite the “Go” signal, it is desirable to provide the same sight distance criteria as applied to stop sign control.
Adequate vertical sight distance is also vital at streets intersecting at sag and summit curves. It is important where physical separations, islands, or other channelization devices are used. Warning signs or other warning devices are desirable but should be depended upon only where corrective sight distance measures are impractical.
Care should be taken to avoid the creation of an illusion at an intersection approach. For example, a traffic circle might not be seen in advance if it is in a small dip in the profile; to a motorist approaching the intersection the road might appear to go straight through. This illusion is sometimes strengthened if a utility pole line runs straight through the intersection or at night if the head- lights of approaching traffic are directly ahead.
E 559.1 Right-Angles Intersections
Figure E 659.1, Plate I, illustrates the minimum sight triangle which should be clear of obstructions where there is no stop or signal control at the intersection. Ideally, the distances 
and 
should be equal to the minimum stopping sight distances for the respective design speeds of the two vehicles. Where it is not economically feasible to remove an obstruction to the sight line, the speed on one of the streets may be regulated by appropriate signing. Assuming that vehicle A on the primary street is traveling at the design speed of that street, then by using the formulas in Section E 442, Safe Stopping Distances, the stopping distance for vehicle A can be determined. The critical stopping distance of vehicle can be evaluated in terms of the distances to the known obstruction in the line of sight. The triangle proportion in this ease is:
Where a, b = Known distances to the obstruction in the line of sight (measured from the paths of vehicle A and vehicle B).
=Minimum safe stopping distance of
=Minimum distance available for vehicle B to stop.
By using the formula for the safe stopping distance as covered above, the corresponding safe speed for the calculated minimum distance available for vehicle B to stop can be determined. The design speed of vehicle A is known (
) and the distances to the sight obstruction from the respective paths of vehicles A and B are known (a and b). The critical speed 
can be evaluated in terms of these known factors. Distance 
is the minimum stopping distance for vehicle A. When vehicle A is at a distance 
from the intersection and the drivers of vehicles A and B first sight each other, vehicle B is at a distance of from the intersection. Using the triangle proportion above, the critical speed of is that for which the stopping distance is . Therefore, the proportion to use in this case is:
rounded off to the nearest 5 miles per hour and the nearest 25 feet.
Where 
.= Design speed of vehicle A in mph (primary street)
= Critical speed of vehicle B in mph (minor street).
The signs on the minor street showing the speed with which to approach the intersection should be so located that the driver of vehicle B can reduce his speed to by the time he reaches the point that is at a distance 
from the intersection. Similar calculations may be used to determine how far back an obstruction needs be moved to provide sufficient sight distance for driving at desired vehicle speeds on the respective streets.
E 559.2 Skew-Angled Intersection
The effect of skew in the angle of intersection (less than 60 degrees) on the sight distance is illustrated in Figure E 659.1, Plate II. Where obstructions at oblique intersections limit sight distance, the distances a and b in the calculations in Subsection E 659.1 should be measured parallel to the intersecting streets.
E 559.3 Stop Control Devices
At cross streets controlled by stop or yield signs, as illustrated on Figure E 659.1, Plate III, sufficient sight distance at the intersections should be provided to permit safe crossing of the primary street. The driver of a stopped vehicle should see enough of the primary street to be able to cross before a vehicle on the primary street reaches the intersection. The required sight distance along the primary street can be expressed as:
Where d = Minimum sight distance along the primary street from the intersection, in feet.
V = Design speed on the primary street, in miles per hour.
= Time required to accelerate and traverse the distance S, in seconds.
For “” reference is made to the graph shown on Figure E 659.3, below.
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