Repeat Problem 3-23 for a highway with a downgrade of –3.5% and the speed allowed on the diversion is 15 mph. Assume that a driver can read a road sign within his or her area of vision at a distance of 40 ft for each inch of letter height.
What will be an ideal response?
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) × 40 feet / inch of letter height
Readability = 8 inches × 40 feet / inch
Readability = 320 feet
The sign can be read at a distance of 320 feet.
Next, determine the distance from the diversion the sign should be placed.
x = SSD – readability distance
x = 662.15 – 320
x = 342.15 ft
The sign should be placed approximately 345 feet prior to the diversion to alert drivers of the change on the highway.
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