How many lanes will be required to provide LOS C for the uphill direction?  If we assume the same traffic and design components, will the downhill lane requirement be the same?

[Solution Shown Below]

Solution

We want to solve the following equation:

N = V /(vP x PHF x fHVx fP)

Where:
V = hourly volume (vph) = 3,080
PHF = peak-hour factor = 0.95
fHV = heavy-vehicle adjustment factor  (equation shown below)
fp = driver population factor = 1.0 for commuter traffic

(for ET and ER use the applicable passenger-car equivalent tables)

For the uphill section,

For the downhill section,

The value for vp can be interpolated from the table or the graph given in the module entitled "Level of Service Criteria and Capacity" after we've adjusted the free flow speed.

Adjusted Free-flow speed =

FFS = 70 – fLW – fLC – fN - fID

Where

fLW = adjustment for lane width = 0
fLC = adjustment for right-shoulder lateral clearance = 0
fN = adjustment for number of lanes (we'll assume 3 lanes to begin with and come back to check to see if it agrees with our final solution) = 3.0
fID = adjustment for interchange density = 5
FFS = estimated free-flow speed = 70 – 0 – 0 – 3.0 – 5 = 62

By interpolation, the maximum service flow rate (vp) for LOS C at a free-flow speed of 62 mph is 1480 pcphpl.

Therefore, for the uphill section:

N = 3080 /(1480 x 0.95 x 0.68 x 1) = 3.2 or 4 lanes

Checking the free-flow speed: 70 – 0 – 0 – 1.5 – 5 = 63.5

And the maximum service flow rate for LOS C for a free-flow speed of 63.5 mph is approximately 1520 vph.  Let's confirm that 4 lanes is still appropriate:

N = 3080 /(1520 x 0.95 x 0.68 x 1) = 3.1 or 4 lanes (always round up).

For the downhill section, and going back to the 3 lane assumption because the heavy vehicle adjustment factor is quite a bit larger: