Question

A six-lane freeway with three lanes in each direction has regular weekday users. The

lanes are 12 feet wide, the right-side shoulder is 2 ft wide, and there are two ramps

within 3 miles upstream and two ramps within 3 miles downstream of the segment

midpoint. The highway is on rolling terrain with 12% large trucks and buses and 2%

recreational vehicles. The peak hour factor is 0.85. Determine the maximum hourly

volumes that can be sustained at LOS C and LOS D densities.

Answer 1

Assume Base free flow speed (BFFS) = 75 mph

Lane width = 12 ft

Reduction in speed corresponding to lane width, f_LW = 0 mph

Lateral Clearance = 2 ft

Reduction in speed corresponding to lateral clearance, f_LC = 1.6 mph

Interchanges/Ramps = 4 / 6 miles = 0.67 /mile

Reduction in speed corresponding to Interchanges/ramps, f_ID = 1.3 mph

No. of lanes = 3

Reduction in speed corresponding to number of lanes, f_N = 3 mph

Free Flow Speed (FFS) = BFFS – f_LW – f_LC – f_N – f_ID = 75 – 0 – 1.6 – 3 – 1.3 = 69.1 mph

PHF = 0.85

Trucks and Buses = 12 %

RVs = 2 %

Rolling Terrain

f_HV = 1/ (1 + 0.12 (2.5-1) + 0.02 (2.0-1)) = 1/1.22 = 0.82

f_P = 1.0

V = Hourly volume

Peak Flow Rate, V_p = V / (PHV*n*f_HV*f_P)

Density = V_p/S

S = FFS

a) LOS C:

Density of LOS C should lie between 18 – 25 veh/mi/ln

Maximum Density = 25 veh/mi/ln

Peak flow rate, V_p = Density * S = 25 * 69.1 = 1728 veh/hr/ln

Maximum hourly volume, V = V_p * (PHV*n*f_HV*f_P) = 1728 * 0.85 * 3 * 0.82 * 1.0 = 3613.25 ~ 3614 veh/hr

b) LOS D:

Density of LOS D should lie between 26 – 35 veh/mi/ln

Maximum Density = 35 veh/mi/ln

Peak flow rate, V_p = Density * S = 35 * 69.1 = 2419 veh/hr/ln

Maximum hourly volume, V = V_p * (PHV*n*f_HV*f_P) = 2419 * 0.85 * 3 * 0.82 * 1.0 = 5058.129 ~ 5059 veh/hr