Estimating capacity impacts of CAVs: A look ahead at HCM 7th Edition (2022) from practitioners’ perspective

Connected and Automated Vehicles (CAVs) are a developing technology that holds great promise in reducing delays, crashes, and fuel consumption. CAVs can communicate with each other and with the roadside infrastructure, allowing their driving systems to navigate and make decisions more efficiently and safely than a conventional human driver.

Given the early stage of CAVs deployment, the scarcity of field data requires multiple assumptions when modeling CAV impacts on highway capacity, which leads to projections ranging from very conservative to very optimistic. This results in a myriad of scenarios that pose a challenge for public agencies who need to plan for the deployment of CAVs over the following decades to develop their long-range transportation plans and make policy and investment decisions.

The Transportation Research Board (TRB)?will release the upcoming 7th Editon of the Highway Capacity Manual which will present, for the first time, methods to evaluate capacity effects of CAVs in the freeways, signalized intersections, and roundabouts. This is an invaluable resource for agencies that now can rely on a nationally recognized, peer-reviewed reference manual for their long-range planning-level decisions. The upcoming Highway Capacity Software (HCS2022) incorporates HCM7 methods on CAVs analyses and will provide a user-friendly environment for practitioners to use these methods for long-range planning.

HCM 7 methods on CAVs

The new methods can estimate capacity improvements as a function of the Market Penetration Rate – the percentage of vehicles in the traffic stream with CAV capabilities. Higher penetration rates lead to higher capacity increases, which is consistent with research findings.

The HCM 7th Edition will provide capacity adjustments for CAVs on freeways, signalized intersections, and roundabouts. These methods are intended for planning-level purposes and can measure CAV effects through adjustments in the computed capacity. Service Volume Tables are also provided allowing for quick estimates of traffic volumes that can be serviced for given sets of road characteristics and a target LOS.

CAV adjustment – Freeways

CAV effects on freeways are measured through a CAF applied to adjust the capacity of a given segment [1]. Based on market penetration rate, specific CAV tables are provided for basic, merge/diverge, and weaving segments. As an example, with a 60% penetration rate of CAVs in the traffic stream, we can expect to gain 13% more capacity on a basic segment with a free-flow speed of 70 mi/h

CAV adjustment – Signalized Intersections

The Signalized Intersections method will address the effects of CAVs by increasing the value of saturation flow rates as a function of the market penetration rate, with different adjustments applicable for the base saturation flow rate and protected and permitted movements. For example, with an 80% market penetration rate, the base saturation flow rate is expected to increase by 21%.

CAV adjustment – Roundabouts

CAV effects on the roundabout are modeled through adjustments on gap acceptance parameters that directly affect capacity: critical headway and follow-up headway. Higher rates of market penetration should allow vehicles to accept smaller gaps and use them more efficiently, improving capacity values. For example, if there are 40% CAVs in a single-lane entry leg with a conflicting flow of 500 pc/h in a single circulating lane, the entry leg capacity will increase by 14%.

Considerations Needed

HCM provides a robust framework to apply methods both for planning and operational level analyses. The CAV impact on capacity is meant to be used for planning level analyses even though it may be used for operational level analysis in some cases. For example, the adjustment on the saturation flow rates could be used for signal coordination purposes for certain peak hour demand profiles for different movements. These adjustments can give an insight into how much capacity is gained due to a higher saturation flow rate.

Conclusions?

The HCM is the first widely recognized reference document to provide CAVs impact on various facility types. The Highway Capacity Software (HCS) will be ready to allow users to confidently model such a novel methodology, assuring accurate analyses that are faithful to the new HCM methods and facilitating the use of results and insights in the planning process of public agencies.

References

1. Adebisi A, Liu Y, Schroeder B, et al. Developing Highway Capacity Manual Capacity Adjustment Factors for Connected and Automated Traffic on Freeway Segments.?Transportation Research Record. 2020;2674(10):401-415. doi:10.1177/0361198120934797