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Calculated FFS < 42.5 mph

SDDOT Rapid City Region
South Dakota Dept. of Transportation
Calculated FFS < 42.5 mph was posted on November 20th, 2014 at 5:08 PM EST
regarding Chapter 14: Multilane Highways

For a 40 mph multilane highway, the BFFS is suggested to be 47 mph. If the section of highway has 11-foot lanes and 20 access points/mile, then the calculated FFS = 47-1.9-5=40.1 mph. This FFS is outside of the lower limits of the ranges given on page 14-3 for which FFS curve to use. Does this mean that highway's LOS cannot reliably be determined from the Ch.14 methodology? Or can I interpolate a 40.1 mph FFS curve into Exhibit 14-5? Thank you!

Gustavo Riente de Andrade
TECTRAN - T├ęcnicos em Transportes ltda
RE: Calculated FFS < 42.5 mph was posted on November 23rd, 2014 at 9:03 PM EST

Yes, that means this section you are calculating is out of the range that HCM was originaly calibrated. However, I would still extrapolate the curves. Notice that the lower the FFS, the lower are the capacities and thus the length of the curve portion of the speed-flow relationship, in manner that it becomes almost a straight line at some point.

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