Common and differing aspects of the SIDRA and HCM models for Two-Way Stop Control (TWSC) capacity and performance estimation are discussed below. Right-hand traffic rule is assumed in this discussion.
Important common aspects of SIDRA and HCM TWSC capacity and performance models include:
- gap acceptance method with consistent values of Critical Gap (Headway) and Follow-up Headway parameters (some minor differences may exist),
- the same equation for basic shared lane capacity formula: the shared lane capacity estimated by SIDRA is calculated from a well-known equation (SIDRA User Guide Section 8.1.13) which is also used by the HCM (Chapter 20, Equation 20-41 for major road left-turn and U-turn (Rank 2) movements in a shared lane, and Equation 20-59 for minor road movements (Rank 3 and 4) in a shared lane), and
- capacity and delay equations (the HCM equations are used in the HCM software setup of SIDRA by default but they are applied to lanes rather than movements).
However, because the SIDRA capacity and performance model for TWSC intersections is a lane-based model compared with the HCM lane-group based model, some fundamental differences exist between the SIDRA and HCM methods for TWSC intersections:
- SIDRA starts with capacity and performance (delay, queue length, stop rate) calculations for each lane and then disaggregates the results to movement delays and movement stop rates (queue length does not get disaggregated since per movement queue length is not useful). In this process, the model takes into account individual movement characteristics according to the movements in each lane including estimated lane flows for movements using more than one lane.
- On the other hand, HCM starts with capacity and delay calculations for each movement and aggregates movement delays to approach values. It also calculates some queue length values per movement. HCM does not give results for:
- lane capacities (including the case when through and right turn (Rank 1) movements are interrupted by a major road opposed left turn (Rank 2) movement in a shared lane),
- lane delays or queue lengths,
- stop rate estimates for movements or lanes,
- back of queue results for movements or lanes (it gives cycle-average queue per movement as stated below Equation 20-68): “The mean queue length is computed as the product of the average delay per vehicle and the flow rate for the movement of interest.”
The back of queue and cycle average queue differ substantially (SIDRA User Guide Section 8.1.6).
There are also differences in some detailed aspects of the SIDRA and HCM TWSC capacity and performance models. These include the following:
- Some factors used in the HCM opposing (conflicting) flow calculations (Equations 20-4 to 20-29) are not used in the SIDRA capacity model as it is assumed that opposed vehicles give way to all opposing (conflicting) vehicles. The only exception is minor road right turn (Rank 2) movements yielding to major road through (Rank 1) movements where SIDRA uses "Percent Opposed by the Nearest Lane Only" rather than 50% of the Through flow used by the HCM.
- The HCM delay equation uses movement capacity values which may be problematic when a movement uses more that one lane where some of the lanes used are shared lanes.
- HCM calculates the delay to through and right turn (Rank 1) movements interrupted by a major road opposed left turn (Rank 2) movement in a shared lane using Equation 20-65. The method used in SIDRA for this purpose is different. It starts by calculating the capacity and delay for the shared lane. It then calculates the delays to the Rank 1 movements according to the delay characteristics of the Rank 2 movement.
- Short lane modelling in SIDRA (User Guide Section 8.1.3) differs from the flared intersection treatments in the HCM (Equations 20-60 to 20-63).