Free-Flow Speed Adjustment

Free-Flow Speed Adjustment

Determining the LOS for a basic freeway section involves two steps:

1) Adjusting a count or estimate of the hourly volume of vehicles to account for the effects of prevailing traffic conditions. See the module "Determining Flow Rate" for information on Step One.

2) Adjusting the free-flow speed for the prevailing design conditions of that section. This module addresses the second step.

The free-flow speed of a freeway section can be obtained directly by field measurement. If field measurements are not feasible, the free-flow speed can be estimated by the following equation, which accounts for the effects of physical characteristics:

FFS = 70 – fLW – fLC – fN - fID

Where

FSS = estimated free-flow speed
fLW = adjustment for lane width
fLC = adjustment for right-shoulder lateral clearance
fN = adjustment for number of lanes
fID = adjustment for interchange density

The adjustment factors can be obtained from the tables below.

Table 1. Adjustment Factors for Lane Width
Lane Width (ft)	Reduction in Free-Flow Speed, f_LW(mph)
>= 12	0.0
11	2.0
10	6.5

Table 2. Adjustment Factors for Right-Shoulder Lateral Clearance
Right Shoulder Lateral Clearance (ft)	Reduction in Free-Flow Speed, f_LC(mph)
	Lanes in One Direction
	2	3	4
>=6	0.0	0.0	0.0
5	0.6	0.4	0.2
4	1.2	0.8	0.4
3	1.8	1.2	0.6
2	2.4	1.6	0.8
1	3.0	2.0	1.0
0	3.6	2.4	1.2

Table 3. Adjustment Factors for Number of Lanes
Number of Lanes (One Direction)	Reduction in Free-Flow Speed, f_N (mph)
>=5	0.0
4	1.5
3	3.0
2	4.5

Table 4. Adjustment Factors for Interchange Density
Interchanges per Mile	Reduction in Free-Flow Speed, f_ID (mph)
<=0.50	0.0
0.75	1.3
1.00	2.5
1.25	3.7
1.50	5.0
1.75	6.3
2.00	7.5