LOS ANGELES • Volkswagen has shown how easy and tempting it can be for car manufacturers to rig the pollution controls on vehicles to cheat the system.

We shouldn't have been surprised. Manufacturers of heavy-duty diesel engines were caught doing the same thing in the 1990s.

How do we guard against this happening again?

It would be a mistake to invest millions of dollars only in improved emissions tests and vehicle computer systems. If computers could be rigged once, they can be rigged again.

What's really needed is a truly independent emissions-testing system that measures pollution where it occurs, on the open road, and not just in a laboratory or emissions testing station.

In fact, this technology already exists.

Remote sensing devices on the roadside can measure emissions as a vehicle passes by, without impeding traffic flow, often without the driver or vehicle knowing they have been tested, and without the vehicle owner having to wait in line at an inspection station.

A single roadside remote sensing device can capture thousands of vehicle emissions measurements a day in free-flowing traffic. These machines use infrared scanning technology to measure emissions, speed and acceleration. A camera records the licence plate number, which can be matched to state vehicle registries.

In the United States, this technology has been around for years. About a half-dozen states now use it routinely to supplement their inspection programmes, and at least 10 others perform periodic surveys and studies, mostly in urban areas with air-quality problems, to monitor overall compliance to clean air rules. In Colorado, for instance, cars that are found in compliance by a remote sensing device are exempted from vehicle emissions tests.

What makes this technology particularly useful is its ability to aggregate emissions data on makes and models of cars and measure how various models, vehicle technology classes and emissions control components are performing on the road.

The results can be eye-opening. I was part of a team of scientists in Colorado that used this technology to identify emissions problems with Volkswagens and Audis that have two-litre diesel engines months before the recent scandal broke.

The first hint came from a colleague in Europe who, looking at remote sensing data collected in Switzerland, had noticed high diesel nitrogen oxide emissions coming from passenger cars. At his suggestion, we examined thousands of measurements collected by Colorado's vehicle emissions programme.

Sure enough, late last year, we found nitrogen oxide emissions from Volkswagen and Audi two-litre diesel vehicles significantly above not only the regulations, but also above the emissions of similar vehicles. But we had no idea then that Volkswagen had rigged inspection tests, let alone of the scope of the company's subterfuge.

Cops walking their beats notice things and, in this sense, remote sensing is the "cop on the beat" of emissions control, spotting abnormalities, defective emissions devices, deteriorating emissions control systems or unexpected emissions in unusual conditions, such as high elevation and high temperature.

When measuring millions of vehicles, remote sensing technology is far less expensive to implement than laboratory testing or other on-road emissions testing methods.

Virginia motorists, for instance, will spend about US$5 million (S$7 million) a year on an expanded remote sensing device programme in the northern part of the state starting soon that will test their vehicles on the road (motorists will be mailed an invoice to cover the cost of the inspection).

The state will allow up to 30 per cent of the cleanest cars to bypass testing at an emissions inspection station. The programme helps the state meet federal air pollution standards by also identifying vehicles emitting high levels of pollution so repairs can be made before the next inspection is due.

Still, overall, not enough is being invested at the state or federal levels to expand these real-world patrols.

The Environmental Protection Agency should establish a network of roadside devices that would monitor vehicles more methodically for abnormal emissions. This would help the agency better understand how cars and trucks are performing on the roads in all conditions.

In the worst cases, the data could provide the probable cause for focused investigations of particular models that are failing to meet emissions requirements. If necessary, any inquiry could be supplemented by the use of so-called portable emissions measurement systems (the vehicle-mounted "lab in a box" used by the investigators from West Virginia University who exposed the Volkswagen cheating).

Given the scope of the Volkswagen case, with more than 11 million cars involved, there are sure to be calls for laboratory emissions tests that can't be thwarted, and for improved vehicle computer systems.

But we shouldn't lose sight of what is really happening on the road - where vehicles actually pollute, and where they can be monitored without detecting that they are being tested. This is why we should significantly expand the use of remote sensing devices, a technology the cheaters can't cheat.

NEW YORK TIMES

• Peter M. McClintock is an air quality and vehicle emissions consultant for Opus Inspection, an international vehicle inspection company, and a contractor for the Colorado and Virginia emissions inspection programmes.