Assessing the Effects of Automated Vehicles (AVs) on Freeway Operations

Many assume that one of the benefits of AVs will be increased vehicular capacity and reduced delay on freeways. Fehr & Peers conducted our own research to test that hypothesis using traffic simulation models of varying conditions of congestion levels and AV fleet mix.

The blue vehicles in the video above are a representation of what we modeled, i.e. AVs being introduced into freeway traffic, mixed with human drivers.

Effects on System Performance

As shown below, AVs are expected to reduce delay and improve speeds due to reduced headways and better roadway utilization, with the greatest benefits occurring under highly-congested conditions.  The benefits, however, tend to max out when the AV fleet mix reaches around 50%-60%.  In other words, once AVs become a majority of the fleet, their operational characteristics begin to control the overall flow, thereby causing drivers of other vehicles to conform to their operating style.

(If you’d like to see more technical details, take a look at our findings on change in delay vs AV fleet and change in speed vs AV fleet.)

Effects on Flow Rates and Speeds

As shown below, the average freeway flow rate increases from about 2,300 vehicles per hour per lane (vphpl) without any AVs to about 3,200 vphpl with 100% AVs.  In terms of travel speeds, the 100% AV scenario postpones the point at which “break down” (i.e., oversaturated) conditions occur.  Also, once oversaturated, the 100% AV scenario shows speeds of about 5-10 miles per hour higher than the 0% AV scenario.

Model Parameters: Our traffic operations modeling work utilized the traffic simulation software, Vissim by PTV Group, which included AV options. We examined the vendor’s recommendations and applied our own combinations of car following, lane changing and lane assignments based on current research (more information available – contact us to learn more).

Early Conclusions

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AVs improve freeway network and corridor performance, owing to reduced headways and more predictable, measured reactions to adjacent vehicles.

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The capacity and delay benefits of AVs max out once the fleet mix reaches 50%-60% AVs.

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While AVs do not prevent the eventual breakdown of traffic flow, they do enable somewhat better operations and increased throughput after breakdown occurs.

Considerations for Your Analysis

AVs will very likely impact freeway operations and should therefore be considered in operational analyses that have a long-range time horizon.

The analysis should also consider the likely changes in travel demand (including induced travel) associated with AVs. Please refer to our webpage on AV travel demand for more insight.

Analysis details should be refined over time as more data on AV operating characteristics becomes available.