Question

A long segment of the suburban freeway is to be designed on level terrain. The level segment, however, is followed by a 4.5% grade, 2.0 miles in length. If the DDHV is 2,500 veh/h with 15% trucks (standard mix), how many lanes will be needed on the (a) upgrade, (b) downgrade, (c) level terrain segment to provide for the level of service C? Lane widths and lateral clearances may be assumed to be 12 ft and 6 ft, respectively. Ramp density is expected to be 1.0 ramps/mi. The PHF is 0.92. Good weather, no incidents, no work zones, and regular users of the facility may be assumed.

Answer 1

(a)

Step 1:

Et=3

fhv=1/1+0.15(3-1)=0.77

V=2500 vph

Assume N=3

vp=V/(PHFxNxfhv)

Vp=2500/(0.92x0.77XN)

Vp=2500/(0.92x0.77x3)=1176 pc/hr/ln

FFS=BFFS-flw-flc-TRD0.84

FFS=60-1=59 mph

D=1176/59=19.93 pc/mi/ln (which satisfies LOS=C, 18<LOS C<26)

(b)

Et=1.5

fhv=1/1+0.15(1.5-1)=0.93

Assume N=2

vp=V/(PHFxNxfhv)=2500/(0.92x2x0.93)=1460 pc/hr/ln

FFS=59 mph

D=Vp/S=1460/59=24.74 pc/mi/ln

Provide 2 number of lanes

(c)

Et=1.5

Et=1.5

fhv=1/1+0.15(1.5-1)=0.93

Assume N=2

vp=V/(PHFxNxfhv)=2500/(0.92x2x0.93)=1460 pc/hr/ln

FFS=59 mph

D=Vp/S=1460/74=24.74 pc/mi/ln

Provide 2 number of lanes

Provide 3 for upgrade , 2 for downgrade and level terrain