An elevated expressway goes through an urban area, and crosses a local street as shown in Figure 3.9. The partial cloverleaf exit ramp is on a 2% downgrade and all vehicles leaving the expressway must stop at the intersection with the local street. Determine (a) minimum ramp radius and (b) length of the ramp for the following conditions below:

Question

Maximum speed on expressway = 60 mi/h
Distance between exit sign and exit ramp = 260 ft
Letter height of road sign = 3”
Perception-reaction time = 2.5 sec
Maximum superelevation = 0.08
Expressway grade = 0%

Assume that a driver can read a road sign within his or her area of vision at a distance of 50 ft for each inch of letter height, and the driver sees the stop sign immediately on entering the ramp.

What will be an ideal response?

Answer 1

First, determine the readability of the roadway sign.

Readability = (Letter height in inches) × 50 feet / inch of height

Readability = 3 inches × 50 feet / inch

Readability = 150 feet

Next, determine the speed of the vehicle just prior to it entering the exit ramp (uexit). Modify Equation 3.27 by replacing the second term (braking distance) with equation 3.26. a/g is taken as 0.35.



Next, determine the minimum radius for this exit ramp, using Equation 3.34. For the exit speed of 40 mi/h, the new coefficient of side friction should be fs = 0.16.

R = u2exit / 15(e + fs)

R = (40.13)2 / 15(0.08 + 0.16)

R = 447.34 feet.

Next, determine the length required for this exit ramp.

S = 1.47uexitt + u2exit / 30 ((a/g) ± G)

S = 1.47(40.13)(2.5) + (40.13)2 / 30(0.35 - 0.02)

S = 310.15 feet

Therefore, the minimum radius for this exit ramp is approximately 450 feet and the minimum length of the exit ramp was found to be approximately 310 feet.