Question

A six-lane freeway (three lanes in each direction) operates at capacity during peak hour. It has 11-ft

lanes, 4-ft right shoulders, and there are three ramps within three miles upstream of the segment

midpoint and four ramps within three miles downstream of the segment midpoint. The freeway has an

AADT of 60,000 veh/day. The traffic stream has 8% heavy vehicles, and it is on rolling terrain with a

peak- hour factor of 0.85. It is known that 12% of the AADT occurs in the peak hour and that the

directional factor is 0.6. What is the freeway’s LOS?

Answer 1

Assume Base free flow speed (BFFS) = 70 mph

Lane width = 11 ft

Reduction in speed corresponding to lane width, f_LW = 1.9 mph

Lateral Clearance = 4 ft

Reduction in speed corresponding to lateral clearance, f_LC = 0.8 mph

Interchanges/Ramps = 7/6 miles = 1.17 /mile

Reduction in speed corresponding to Interchanges/ramps, f_ID = 2.5 mph

No. of lanes in one direction = 3

Reduction in speed corresponding to number of lanes, f_N = 3.0 mph

Free Flow Speed (FFS) = BFFS – f_LW – f_LC – f_N – f_ID= 70 – 1.9 – 0.8 – 3.0 – 2.5 = 61.8 mph

AADT = 60,000 veh/day

Peak hour in AADT = 12%

Directional factor = 0.6

Peak Flow, V = 60,000 * 0.12 * 0.6 = 4320 veh/hr

Peak-hour factor = 0.85

Trucks = 8 %

Rolling Terrain

f_HV = 1/ (1 + 0.08 (2.5-1)) = 1/1.12 = 0.893

f_P = 1.0

Peak Flow Rate, V_p = V / (PHV*n*f_HV*f_P) = 4320/ (0.85*3*0.893*1.0) = 1897.11 ~ 1898 veh/hr/ln

S = FFS

S = 61.8 mph

Density = V_p/S = (1898) / (61.8) = 30.71 vehmi/ln

Hence Level of Service is D

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