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Level Terrain

teri anderson
caltrans
Level Terrain was posted on February 24th, 2012 at 3:31 PM EST
regarding Chapter 11: Basic Freeway Segments

Level Terrain is defined as ". . permits heavy vehicles to maintain the same speed as passenger cars . . ." On California freeways the posted speed for trucks is typically 15 mph lower than passenger cars. Is there a way to account for this other than not choosing level terrain?

Erik Ruehr
VRPA Technologies
RE: Level Terrain was posted on February 26th, 2012 at 11:46 AM EST

On California freeways with level terrain, I typcally use the level terrain parameters with no adjustments on the assumption that the base data has taken lower truck speeds into account (although I don't know for sure that this is true). You could increase the PCE factors for trucks in order to emphasize the effect of lower truck speeds on the capacity calculations.

Gustavo Riente de Andrade
TECTRAN - Técnicos em Transportes ltda
RE: Level Terrain was posted on March 15th, 2012 at 8:25 PM EDT

In Brazil, we also have different posted speed for trucks and automobiles. As a researcher at University of São Paulo, we found out that not much attention have been directed to this factor in past researches. The database used for the calibration of the simulation models which produced the equivalent factor given in the HCM apparently didn't account for this too. Anyway, as posted speed doesn't aways equal the free flow speed, I still prefer to base the terrain classification in geometric parameters of the roadway, rather than considerarions over the truck fleet, although I agree that this would also be a valid approach.

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