A temporary diversion has been constructed on a highway of +4% grade due to major repairs that are being undertaken on a bridge. The maximum speed allowed on the diversion is 20 mi/h. Determine the minimum distance from the diversion that a road sign should be located informing drivers of the temporary change on the highway. Assume that a driver can read a road sign within his or her area of vision at a distance of 30 ft for each inch of letter height.

Question

Speed limit on highway = 65 mi/h
Letter height of road sign = 8 inches
Perception-reaction time = 2.5 sec

What will be an ideal response?

Answer 1

Use equation 3.35 to determine the stopping sight distance.



While the first term in this equation is simply the distance traveled during the perception-reaction time, second term is the distance traveled during braking. a/g is taken as 0.35. Since the vehicle is not stopping (the final speed is not equal to zero), the equation needs to be modified to take this into consideration. The second term of equation 3.35 is replaced by equation 3.26 as follows (in which u1 is the initial velocity and u2 is the final velocity):



Next, determine the readability of the roadway sign.

Readability = (Letter height in inches) × 30 feet / inch of letter height

Readability = 8 inches × 30 feet / inch

Readability = 240 feet

The sign can be read at a distance of 240 feet.

Next, determine the distance from the diversion the sign should be placed.

x = SSD – readability distance

x = 565.79 – 240.00

x = 325.79 feet

The sign should be placed approximately 325 feet prior to the diversion to alert drivers of the change on the highway.