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:
Rp = P/(g/C)
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.