Example $10$

An elevated expressway goes through an urban area, and crosses a

local street as shown in Figure. The partial cloverleaf exit ramp is on a

$6\%$ 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:

Maximum speed on expressway $=60m\frac{i}{h}$

Distance between exit sign and exit ramp $=$

$260ft$

Letter height of road sign $=5"$

Perception$-$reaction time $=2\mathrm{.}5sec$

Maximum superelevation $=0\mathrm{.}08$

Expressway grade $=-6\%$

Assume that a driver can read a road sign

within his or her area of vision at a distance of

$50ft$ for each inch of letter height, and the driveI

sees the stop sign immediately on entering the

ramp.

$SSD=1\mathrm{.}47ut+\frac{u_1^2-u_2^2}{30(\frac{a}{g}+-G)}$