Thanks to a safety standard issued by the National Highway Traffic Safety Administration, electronic stability control (ESC) will be required on all new passenger vehicles starting with 2012 models. The feature is saving lives and reducing insurance losses under collision coverage, but previous analysis by the Highway Loss Data Institute indicates that ESC's benefits vary among vehicles. New research begins to examine whether vehicle-handling tests like the one manufacturers use to certify compliance with the rule requiring ESC can be used to understand the reasons for these differences.
The Institute recently ran obstacle-avoidance maneuver tests with SUVs equipped with stability control. Although vehicle performance in the tests varied, the results didn't correlate with insurance loss patterns, leading researchers to conclude that one test condition by itself can't explain the real-world differences observed in insurance data. Analysis of insurance losses indicates that vehicle and driver characteristics account for much of the variation, suggesting that differences in the ESC systems themselves are small.
ESC helps drivers control their vehicles during high-speed maneuvers like entering curves or swerving to avoid obstacles on slippery highways (see "ESC reduces multiple-vehicle crashes as well as single-vehicle ones," June 13, 2006). ESC senses when a vehicle strays from the intended travel path or begins to spin out. Then the system automatically brakes individual wheels and sometimes reduces throttle to keep the vehicle under control and moving in the intended direction of travel.
The test track research grew out of an analysis published in September 2009 by the Highway Loss Data Institute. HLDI, an affiliate of the Institute, found that ESC reduces losses under collision coverage by 18 percent for 1998-2008 SUVs with ESC compared with predecessor models without it. The reductions weren't uniform across all of the 48 SUVs examined. Results ranged from a 44 percent decrease for the Toyota 4Runner 4-door 4-wheel drive to a 5 percent increase for the Honda Element 4-door 2-wheel drive.
"Our goal was to see if we could zero in on test responses that help explain the insurance loss data," explains David Zuby, the Institute's chief research officer. "If we could, that would help determine if it's possible to enhance current ESC technology beyond what U.S. regulations require."
Engineers picked 8 SUVs with varying insurance loss reductions to subject to tests with ESC turned on and off and examined dozens of metrics. Researchers conducted the work in conjunction with the Transportation Research Center, an independent automotive test center in Ohio.
"We couldn't correlate the test track results with HLDI's loss data by vehicle make and model," Zuby says. "We think most of the differences HLDI found in ESC effectiveness reflect things like how a vehicle handles, its size and weight, and who's at the wheel more than they do the system's design or manufacturer. That is, the SUVs with the highest insurance losses to begin with get the biggest benefit from ESC, not because they have better stability control systems but because they are more likely to get into situations that ESC is designed to prevent."
ESC is so important that it's among the criteria to win Top Safety Pick, the Institute's award for vehicles with state-of-the-art crash protection. It's also a requirement to land on Consumer Reports' recommended list, and the National Highway Traffic Safety Administration recommends ESC as part of the New Car Assessment Program.
Institute research shows that ESC reduces fatal single-vehicle crash risk by 49 percent and fatal multiple-vehicle crash risk by 20 percent for cars and SUVs (see "Stability control reduces fatal crash risk by a third," June 19, 2010). It lowers the risk of a deadly crash by 33 percent overall and cuts the risk of a fatal single-vehicle rollover by 73 percent.
Federal rules require ESC systems to have certain specified components and capabilities, and vehicles must meet performance requirements to prevent oversteer and understeer in a dynamic test. Oversteer happens when the rear of a vehicle begins to slide or spin out. Understeer happens when the front of a vehicle continues to go straight even as the driver steers the vehicle to move right or left. ESC helps in both situations.
During U.S. rulemaking, the Institute and others raised questions about whether the standard might limit ESC advancements. The concern was that the required compliance test for ESC would discourage manufacturers from exploring ways to make the feature even more effective.
"That doesn't appear to be the case because we know ESC is paying dividends as intended," Zuby says. "The government's compliance test is meant to measure the difference in performance between a vehicle with ESC and one without it, and it does a good job of that." What it can't do, Zuby explains, is tell if an ESC system is a strong one or a weak one. "At this point, no single test does that."
Euro NCAP and other groups also are working to see if it's possible to optimize ESC. The U.S. standard is the basis of a Global Technical Regulation on ESC adopted in June 2008 by the United Nations' World Forum for Harmonization of Vehicle Regulations. Besides the United States, ESC is mandatory in Australia, Canada, and the European Union starting with 2012 models.