Lec 27, Ch.7, pp.249-257: Sight distance at intersections (Objectives). Understand the availability of adequate sight distance is crucial to reduce crashes at intersections Learn the sight distances required at intersections are dependent on the type of control
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At signalized intersections, the unobstructed view may be limited to the area where the signals are located. However, this is acceptable because the signal controls the allocation of right-of-way.
For unsignalized intersections, it is necessary to provide and adequate view of the crossroads or intersecting highways to reduce the potential of collision with crossing vehicles.
The sight distance required depends on the type of control at the intersection.
No control: Basic rule-of-the-road control
Assumption: The drivers may slow down or speed up but do not stop (thus avoiding collision, hopefully).
Sight distance for sight triangle
This distance include the distance traveled by the vehicle, total distance traveled during the driver’s perception reaction time (2 sec) and during brake actuation or the acceleration to regulate speed (1 sec). Dist = 1.47ut
These are the minimum distances for various speed levels. AASHTO Greenbook recommends that minimum stopping sight distances (Tab 3.3) be provided.
You can see two similar triangles in this diagram.
If any three of the variables da, db, a, and b are known, the fourth can be determined by this formula. In the field, you measure b and a. Then, you set da or db using the minimum sight distance for the sight triangle and see if the remaining db or da meet the minimum sight distance requirement. Or, the da is first determined for the major road speed; then from the available db, the speed limit of the minor road is determined.
(Review Example 7-2)
Vehicles on the minor road are required to yield to vehicles on the major road Either slow down or stop if necessary.
You must provide minimum stopping sight distances on the minor road (see Tab 3.3) because you need to slow down or stop. Hence, sight distances for the sight triangle (Case I) are NOT used; they are too short.
The typical value of min t (Perception reaction time) for design is 2.5 sec according to the AASHTO “Green Book.”
Crossing the intersection, thereby clearing traffic approaching from both sides of the intersections
Turning left onto the crossroad, which requires clearing the traffic approaching from the left and then joining the traffic stream on the crossroad with vehicles approaching from the right.
Turning right onto the crossroad by joining the traffic on the crossroad with vehicles approaching from the left.
L=Veh. Length (18 to 20 ft typically)
Sight distance required along the major highway from the intersection that must be available for a safe crossing is obtained by the following formula.
D=Distance to stop line (10ft)
Where v = design speed on the major highway (mph)
J = perception-reaction time plus the time required by the driver to actuate the clutch or an automatic shift (2.0 sec is assumed)
ta = time taken to accelerate and clear the intersection.
The ta depends on the type of vehicle (its acceleration capability) and a distance S, which is give as
S = D + W + L
Design vehicle = SU
D = 10 ft
L = 25 ft (Longer than typical 20 ft for a SU)
W = 50 ft
S = D + W + L
= 85 ft
ta = 8.4 seconds
It takes for a SU 8.4 seconds to completely clear the intersection.
Time (ta) required to travel a distance (S)
Sight distance measurement in the field:
(Review Example 7-3.)