The last thing the National Highway Traffic Safety Administration wants to do is create unsafe driving conditions.
But the federal agency, backed into a corner by the Office of Management and Budget, may be doing just that. OMB forced NHTSA to issue a final rule on tire pressure monitoring devices allowing the use of either direct or indirect systems.
Without minimum performance standards for both kinds of systems, the rule could hurt motorists far more than it helps them.
Monitoring systems are mandated on new passenger cars and light trucks beginning in 2003 under the recently enacted Tire Transportation Recall Enhancement, Accountability and Doc- umentation (TREAD) Act.
The two systems are not equally effective in warning drivers of underinflated tires. Indirect systems, cheaper to install and therefore favored by auto makers, fall woefully short when it comes to detecting and alerting drivers of inadequate air pressure in more than one tire.
And human nature being what it is, motorists likely will put aside their tire gauges and come to rely on these systems to warn them when their tires lose pressure and become unsafe. This isn't a problem with the direct system, but it well could be with less-precise indirect monitors.
Direct tire pressure monitors use a sensor on each wheel or embedded in the tire to send a radio signal to a monitor on the car's dashboard or rearview mirror. They can detect when the air pressure in one or more tires falls as little as 1 psi.
In contrast, a typical indirect monitoring system relies on the vehicle's anti-lock brake mechanism to detect differences in wheel rotation speed caused by tire deflation.
Problems arise when two or more tires lose air simultaneously, thereby continuing to rotate at approximately the same speed. Since the wheels are turning at the same rate, the indirect monitoring system doesn't detect the loss in pressure and therefore can't inform the driver.
In a real-world test of an indirect system on his new Pontiac Bonneville, Marvin Bozarth, senior technical consultant for the Tire Industry Association, found it either worked or didn't work depending on how many tires were underinflated.
The system failed to warn when two or more tires were underinflated-even with two front tires deflated to 20 psi and the left rear tire to 15 psi.
``If I had had a full load of passengers and been driving 75 mph, I believe the tire with 15 psi would have failed within five miles,'' he wrote in the August issue of TIA's TireRetreadingRepairing Today magazine.
That's not the situation the public or the auto industry wants. NHTSA must insist such systems work under all conditions.