BFFS

[north6633]

Hi,

I can't find it as a reference in the HCM when you should use the assumes BFFS for a road segment when you are given a design speed. I am referring to question 3 in the trans 6 min solutions. they give a design speed of 60. But if you look in the HCM for a free way segment it says to assume 70 for urban and 75 for rural. I thought his might be one of those little tricks I get tripped on often. But the solution uses the design speed. What is correct. I hate to do a problem totally right and use the wrong assumption.

I could have swore I read about this in the HCM but I have searched to no avail.

Thanks for the help

 

[Road Guy]

I always use 70 (or what the HCM says as the default) unless it gives you the design speed in the problem.

I will look it up but there is a whole chapter in the HCM that goes over how it uses defaults & which defaults it recommends for certain types of roads / terrain (really handy on the exam)

 

[bridge_guy]

the BFFS is the ideal free flow speed (i.e. design speed) then subtract the other variables like lane width factor, etc. If it isnt given then assume 70mph, which is out of the HCM2000. Well thats what I have been doing.