Speed Limit, FFS and BFFS

[Jambruins]

Can someone please explain the diffence in these items for me? I know the FFS = BFFS - adjustment factors. If a problem gives me the design speed as 60mph is this the FFS, the BFFS or the speed limit? Thanks.

 

[rolltyde]

What type of problem?

 

[Jambruins]

I guess just in general. If I need to find the corresponding pc/h/pl for a certain level of service I would need the FFS. If the problem gives me the design speed as 60mph is this the BFFS or the FFS? What if they give me the speed limit as 60mph, is this the FFS or the BFFS? Thanks.

 

[tsli001]

Check HCM it said there trust me

 

[rolltyde]

In the examples I have worked, if FFS or BFFS is not given, then the default values are used and the defaults are based on rural or urban.

 

[Jambruins]

what if they give you the design speed or the speed limit? Is the design speed the same as the BFFS or the FFS? Is the speed limit the same as the BFFS or the FFS? Thanks.

 

[Vishal]

BFFS:

Use BFFS if it is specifically mentioned or the phrase "Ideal Free-Flow Speed" is used instead of BFFS.

FFS:

For all other variants, "Posted Speed Limit", "Field Measured Speed Limit", etc. use FFS.

 

[petobe]

"Design Speed" refers to BFFS

 

[Earlybird]

"Design Speed" refers to BFFS

In a few 6 Minute Solution problems they use "Design Speed" as the BFFS.

 

[petobe]

FFS:For all other variants, "Posted Speed Limit", "Field Measured Speed Limit", etc. use FFS.

i dont think this is true.... see Example 1 in the HCM, chapter 23. They give a posted speed limit of 70 mph, but use a BFFS of 75 (default for rural) and calculate a FFS of 68.2. If the speed limit had been used as the FFS in this problem, it would have yielded an incorrect answer (LOS C instead of B )

 

[Jambruins]

I guess if they give you enough information regarding the adjustment factors then you probably need to determine the FFS by subtracting the adjustment factors from the BFFS. If they don't give you information regarding the adjustment factors then the speed given is probably the FFS.

 

[civilized_naah]

I guess if they give you enough information regarding the adjustment factors then you probably need to determine the FFS by subtracting the adjustment factors from the BFFS. If they don't give you information regarding the adjustment factors then the speed given is probably the FFS.

ntIn my opinion, the FFS, which is calculated by applying location-specific adjustment speed penalties to the ideal, or base free flow speed, is closest in character to the DESIGN SPEED for a particular location on a highway, because the design speed takes into account specific conditions and limitations there (such as inadequate sight distance, narrow lanes etc.). The posted speed limit is usually 10-15 mph less than the design speed. There are quite a few inconsistencies in the solutions to these problems in the 6-minute solutions. The HCM, as far as I know, does not specifically indicate this (what I wrote above). It just provides default values for BFFS. However, for the sake of the PE exam, if the design speed is given, it makes most sense to assume the FFS = design speed (my opinion)

 

[Ambrug20]

Can someone please explain the diffence in these items for me? I know the FFS = BFFS - adjustment factors. If a problem gives me the design speed as 60mph is this the FFS, the BFFS or the speed limit? Thanks.

 

[Ambrug20]

Can someone please explain the diffence in these items for me? I know the FFS = BFFS - adjustment factors. If a problem gives me the design speed as 60mph is this the FFS, the BFFS or the speed limit? Thanks.

In general when you are given 60mph (free flow speed), you need to find an adjusment factors to have a real spead on the road. Use the formula FFS=BFFS minus all the adjustments. (i have older virsion of CERM, eq. 74-20) If BFSS (base free flow spped) not given use defaul data as 1900.