Question

A freeway is being designed for a location in rolling terrain. The expected free-flow speed is 55 mi/h. During the peak hour, it is expected there will be a directional peak-hour volume of 2700 vehicles and 18% heavy vehicles. The PHF is expected to be 0.88. If a level of service no worse than D is desired, determine the necessary number of lanes. (20 points)

Answer 1

Answer:

3.

Explanation:

We start calcualting the factor for heavy-vehicle, that is,

$f_{HV} = \frac{1}{1+P_T(E_T-1)+P_R(E_R-1)}$

$E_T = 2.5$ for rolling terrain, so, replacing,

$f_{HV} = \frac{1}{1+0.18(2.5-1)+0}$

$f_{HV} = 0.7874$

We calculate the number of lanes by using the relation as follow:

$N=\frac{V}{\upsilon_p PHF f_{HV} f_p}$

$N=\frac{2700}{1915*0.88*0.7874*1}$

$N= 2.035$

In this way the necessary number of lanes is 3.

Note: To calculate $\upsilon_p$ you need to consult the Exhibit 23-2

in 'Highway capacity Manual' for 55mph and the LOS D criteria.