Integration of AVs in the I-15 Corridor in San Diego, California Suggests Traffic Flow and Density Improvements.
San Diego, California, United States
Safety Impact Evaluation of a Narrow-Automated Vehicle-Exclusive Reversible Lane on an Existing Smart Freeway
Summary Information
This project was initiated by Caltrans, looking to increase throughput on four HOV lanes (express lanes) on a seven-mile section of the I-15 in San Diego County. The study evaluated the implications of converting the concrete median that separates the express and regular lanes into a nine-foot automated vehicle (AV) exclusive reversible lane. The study conducted an extensive literature review, an AV manufacturers product review, expert interviews, a consumer questionnaire review, a crash data analysis, and a microscopic traffic simulation analysis. The microscopic simulation model, developed in a commercially available software package, was used to evaluate the impact of the exclusive AV lane on traffic conditions under different market penetration rates (MPR).
METHODOLOGY
The traffic simulation modeled three different scenarios:
| Scenario | Description | AV MPR Modeled |
|---|---|---|
| 1 | existing volumes/network | 0 percent |
| 2 | existing volumes with AV adoption on all lanes | varying MPRs of 15 percent, 30 percent, and 45 percent |
| 3 | existing volumes and with the proposed nine-foot AV exclusive lane | varying MPRs of 15 percent, 30 percent, and 45 percent |
Under scenarios two and three, only SAE level 3 AVs (in which the vehicle can perform most driving tasks on its own with the capability of human intervention) were considered. This study estimated the benefits of AVs by the change in traffic flow, density, speed and speed differentials.
FINDINGS
Scenario two did not show any measurable benefits on traffic volume, density, and average speed in this analysis. However, the following benefits were observed when the AVs were considered only in the narrow exclusive lane:
- The introduction of SAE level three (Conditional Driving Automation) AVs on an AV-exclusive lane resulted in up to a 14 percent increase in traffic flow depending on the segment of the corridor.
- The average density of most segments was found to increase by up to 24 percent measured in vehicles per mile per lane.
- The average speed declined by two to eight miles per hour (mph), signifying that AVs are following the speed limit.
- The speed differential (difference between free speed and actual speed) was higher in scenario three, indicating a higher speed reduction. This is considered beneficial as lower speeds can be associated with lower crash severity.
Table 2 shows that higher market penetration rates under scenario three had the highest speed differential.
Table 2 Speed Differential Range (mph)
|
Range |
Scenario 1 |
Scenario 2 AV 15% |
Scenario 2 AV 30% |
Scenario 2 AV 45% |
Scenario 3 AV 15% |
Scenario 3 AV 30% |
Scenario 3 AV 45% |
|
Lower Range |
-0.7 |
-1.9 |
-2.2 |
-1.4 |
-6.0 |
-9.4 |
-14.3 |
|
Upper Range |
1.4 |
0.2 |
0.4 |
0.9 |
-1.9 |
-4.8 |
-6.6 |
