Module 5 : Uninterrupted Flow

Lecture 25 : Ramp Metering

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	Diverge influence area The Diverging influence area is the area where increase in local density, congestion, and reduced speeds is generally observed due to diverging traffic to ramps. The ramp which diverge traffic to the ramp is called an OFF ramp. The analysis of the diverging influence area is done to find out the level of service of the OFF ramp. The analysis of diverge area is done in following three primary steps: Predicting entering flow The first step is same as that of merge area analysis. The flow in lanes 1 and 2 of the freeway is first predicted. However, there are two major differences in the analysis of diverge area. First, approaching flow $V_{12}$ is measured for a point immediately upstream of the deceleration lane. Second, $V_{12}$ includes $V_{R}$ at the diverge area. V12 is the flow rate entering ramp influence area (pc/h), and vR is the Off-ramp demand flow rate (pc/h). Figure 1: Typical diverging area diagram The general model given by HCM 2000 treats $V_{12}$ as the sum of the off-ramp flow plus a proportion of the through freeway flow. $$V_{12}=V_{R}+(V_{F}-V_{R})\times~P_{FD}$$ (1) where, $V_{12}$ is the flow rate in lanes 1 and 2 of freeway upstream of diverge area in (pc/hr), $V_{F}$ is the freeway demand flow rate immediately upstream of diverge in (pc/h), and $P_{FD}$ is the proportion of through freeway flow remaining in lanes 1 and 2 immediately upstream of diverge. For four lanes freeway (2 lanes in each direction) $P_{FD}$ is 1.00. Determining capacity As in the merge area analysis, determining the capacity is the second step of the diverge area analysis. Three limiting values should be checked: Total flow that can depart from the diverge: this is limited by the capacity of the lanes in the freeway prior to approach of the diverge. The capacities of the departing freeway leg or legs or ramp or both. This is the most important of the three as generally diverge areas fail due to failure of one or more exit legs.. $V_{12}$ (approaching flow) prior to deceleration lane: this flow also includes the off-ramp flow and must be checked against capacity. Determining LOS Determine the level of service (LOS) of the diverge area is the third step of the diverge area analysis. LOS criteria for diverge area are based on density in the diverge influence area. HCM 2000 provides the equation to estimate the density in the merge influence area. $$D_{R}=a+b~V_{12}+c_{LD}$$ (2) where, $D_{R}$ is the density of diverge influence area (pc/km/ln), $V_{12}$ is the flow rate entering ramp influence area (pc/h), $L_{D}$ is the length of deceleration lane(m), and a, b & c are constants. This equation is applicable only for under saturated conditions of flow. The density calculation is not done when either of the three capacities mentioned earlier are exceeded. In such cases, the LOS is assigned as F.