Coordinated Intersections in HCM 2010

The 2010 Highway Capacity Manual (HCM) procedures may be used to analyze coordinated intersections, but there are some unique settings such as the Platoon Ratio required for accurate results. Chapter 18 in HCM 2010 highlights the procedures related to isolated signalized intersections, while Chapter 17 focuses on the procedures associated with coordinated intersections. The Platoon Ratio describes the quality of progression for a platoon of vehicles arriving on green. As described in HCM 2010, the user should select a Platoon Ratio based on field observations and their knowledge of the corridor.

Table 1: Traffic Flow Along Coordinated Arterials

Table 1 includes a brief description of the HCM 2010 platoon ratios.

Synchro already models coordinated signals and therefore does not require additional data to calculate the Platoon Ratio. The user should, however, be sure to code the following settings located in the Synchro Timing Settings window:

• Reference Phase: Select the phase(s) from which offsets should be calculated. Many controllers refer to this as the coordinated phase(s). This is usually the phase for the main street. Phases 2 and 6 are commonly assigned to the main street and used as the coordinated phases.
• Offset: The current offset for the intersection is the amount of time in seconds that the reference phase begins (or ends) after the nearby adjacent intersections.
• Reference to: Select the point from which offsets should be referenced. Two of the most common are the Beginning of Green and Beginning of Yellow.

Synchro automatically calculates a Platoon Ratio for approaches of a coordinated intersection based on the Reference Phase. The following criteria are required to calculate a Platoon Ratio:

• Upstream signalized coordinated intersection within 0.60 mile
• Upstream intersection and study intersection must have equal, double, or half Cycle Lengths
• Reference phase of upstream intersection points towards study intersection

Approaches that don’t meet these criteria are assumed to have a random arrival pattern and are assigned a Platoon Ratio of 1.00. You can override the calculated value based on field conditions. Platoon Ratio may be estimated from field data using the equation:

   Rp = P/(g/C)

    where:

    Rp = platoon ratio

    P = proportion of vehicles arriving during the green indication (decimal)

    g = effective green time (s)

    C = cycle length

In many cases, the platoon ratio will have a significant impact on the resulting delay and queue length. Users are encouraged to review this value carefully.

To demonstrate the relationship between vehicles arriving on green and the platoon ratio, let’s consider vehicles traveling along Main Street (see image below) and observe the flow patterns shown on the time-space diagram. The calculated values for the eastbound (EB) and westbound (WB) platoon ratios at 2nd, 3rd, and 4th Street have also been included. This allows one to visualize the relationship between arrival on green and the platoon ratio.

In the Time-Space Diagram, two points of interest have been highlighted for discussion purposes. First, let’s review vehicles traveling eastbound (EB) at 2nd Street. The platoon arrives at 3rd Street at the beginning of the EB through phase, thus allowing the vehicles to continue traveling without stopping. Since these vehicles experience excellent progression, a Platoon Ratio of 2.00 is assigned. Now let’s look at the westbound (WB) group of vehicles traveling from 4th Street. These vehicles arrive at 3rd Street during a red indication, thus creating a stop for all vehicles (and inducing delay). Since all of the vehicles arrive on red, a Platoon Ratio of 0.33 is assigned. Additional values of the Platoon Ratio have been included to allow you to review additional flow patterns.