Module 5 : Uninterrupted Flow
Lecture 23 : Multilane Highways
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Determination of Level of Service

The determination of level of service for a multilane highway involves three steps:
  1. Determination of free-flow speed
  2. Determination of flow rate
  3. Determination of level of service

Free-flow speed

Free-flow speed is the theoretical speed of traffic density, when density approaches zero. It is the speed at which drivers feel comfortable travelling under the physical, environmental and traffic conditions existing on an uncongested section of multilane highway. In practice, free-flow speed is determined by performing travel-time studies during periods of low-to-moderate flow conditions. The upper limit for low to moderate flow conditions is considered 1400 passenger cars per hour per lane(pc/h/ln) for the analyses. Speed-flow and density flow relationships are shown in Fig. 1 and Fig. 2. These relationships hold for a typical uninterrupted-flow segment on a multilane highway under either base or no base conditions in which free-flow speed is known. Fig. 1 indicates that the speed of traffic volume up to a flow rate of 1400 pc/h/ln. It also shows that the capacity of a multilane highway under base conditions is 2200 pc/h/ln for highways with a 90 km/h free-flow speed. At flow rates between 1400 and 2200 pc/h/ln, the speed on a multilane highway drops; for example, by 8 km/h for a highways with a free-flow speed of 90 km/h. Fig. 2 shows that density varies continuously throughout the full range of flow rates. The capacity value of 2200 pc/h/ln is representative of the maximum 15-min flow rate that can be accommodated under base conditions for highways with 90 km/h free-flow speed.
Figure 1: Speed-flow relationship on multilane highways
\begin{figure} \centerline{\epsfig{file=qfSpeedFlow.eps,width=8 cm}} \end{figure}
Figure 2: Density-flow relationships on multilane highways
Figure 3: Speed-flow curves with LOS criteria for multilane highways
\begin{figure} \centerline{\epsfig{file=qfDensityFlow.eps,width=8 cm}} \centerline{\epsfig{file=qfSpeedFlowCurve.eps,width=8 cm}} \end{figure}
From various studies of the flow characteristics, base conditions for multilane highways are defined as follows:
  1. Lane widths are 3.6 m.
  2. Lateral clearance is 1.8 m.
  3. A minimum of 3.6 m of total lateral clearance in the direction of travel. Clearances are measured from the edge of the outer travelled lanes (shoulders included) and lateral clearance of 1.8 m or greater are considered to be equal to 1.8 m.
  4. No direct access points along the highway.
  5. A divided highway.
  6. Only passenger cars in the traffic stream.
  7. A free-flow speed of 90 km/h or more.
Figure 4: Flowchart showing step by step procedure to find density and LOS
\begin{figure} \centerline{\epsfig{file=qfSpeedFlowLOS.eps,width=8 cm}} \end{figure}
The average of all passenger-car speeds measured in the field under low volume conditions can be used directly as the free-flow speed if such measurements were taken at flow rates at or below 1400 pc/h/ln. No adjustments are necessary as this speed reflects the net effect of all conditions at the site that influence speed, including lane width, lateral clearance, type of median, access points, posted speed limits, and horizontal and vertical alignment. Free-flow speed also can be estimated from 85th-percentile speed or posted speed limits, research suggests that free-flow speed under base conditions is 11 km/h higher than the speed limit for 65 km/h to 70 km/h speed limits and 8 km/h higher for 80 km/h to 90 km/h speed limits. Fig. 4 shows speed-flow curves with LOS criteria for multilane highways, here LOS is easily determined for any value of speed simply by plotting the point which is a intersection of flow and corresponding speed. Note that density is the primary determinant of LOS. LOS F is characterized by highly unstable and variable traffic flow. Prediction of accurate flow rate, density, and speed at LOS F is difficult.

Table 1: Level of Service criteria for a typical free flow speed of 100 kmph proposed in HCM 2000
Free-Flow Criteria (LOS) (LOS) (LOS) (LOS) (LOS)
Speed   A B C D E
100 km/h Max. density 7 11 16 22 25
  (pc/km/ln)          
  Average speed 100 100 98.4 91.5 88
  (kmph)          
  Max. volume 0.32 0.50 0.72 0.92 1.00
  capacity ratio          
  Max. service 700 1100 1575 2015 2200
  flow rate          
  (pc/h/ln)