How does this drive-by emissions test work? (Northern Virginia)

if there’s anything trivially measurable other than CO2 and water coming out of your tailpipe, your car has a problem.

Tell it to Volkswagen, though – if you are not even going to be tested or your only test wil be a sampling at the factory, people may be motivated to cut corners; and in general when health/safety rules are loosened, I wonder how much potential profit or savings it represents for the industry vs. any “tax on the poor” (who anyway benefit from a cleaner environment) effect. It is true that newer vehicles are lower-emission by design, not requiring regular preventive tuning up to keep them compliant for a long time, but at some point it becomes needed, and if it’s never detected that the vehicle is failing to comply, it will stay on the road a lot longer while noncompliant.

Minnesota did that quite a few years ago.
The emissions testers were reporting that more than 95% of vehicles were passing the test, so it was hardly worth the cost of maintaining a string of test centers, and the cost of DMV workers to deal with the extra paperwork. Even the first year it was required, over 80% of vehicles passed.

It was especially pointless since, by Minnesota law, vehicles older than 12 years were exempt from the testing. And you could get 1-year waiver if repairs would cost over $200. And it only applied in the metro area, so if you drove out into the rural area and licensed your vehicle there, you didn’t have to have it tested.

The state of Washington finally did something right. Starting this Wednesday, no more emission testing in the state. The failure rate is so low now someone figured it was a waste of time and taxpayers money.

After decades of mandatory emissions testing for older cars, the province of Ontario abandoned its stupid emissions-testing program, too. I am an environmental advocate, but stupid is stupid. Most mechanics hated it as a useless tax grab, and my own mechanic hated it to so much he refused to have anything to do with it and it was generally wise not to even bring up the subject with him. Newer cars were exempt anyway, old historic cars were exempt, and the only cars that had to pass the OBD II tests every two years were those that almost invariably passed anyway, increasingly so as the average car on the road became newer with more modern technology. And if they didn’t pass, and the repair was exorbitantly expensive, then the polluting piece of junk would get a “conditional pass” anyway. The whole system – along with some of the insanely strict rules about safety checks when reselling a car – seemed intended to simply put a lot more new cars on the road, which is hardly environmentally friendly when you consider the carbon footprint of manufacturing a whole new car.

To avoid criticism of a tax grab, a few years ago the Ontario government made the inspections free – that is, the government paid for them. Which didn’t make the mechanics happy, either, presumably because they were getting lower rates. One guy had a big sign up: “Nothing is free – it’s all paid for with YOUR tax dollars!” Now, thankfully, we’ve seen the end of it, because we don’t need it any more. The only exceptions are heavy big trucks.

The typical location is a highway on-ramp where you are accelerating, i.e. you’re making a significant amount of power (and therefore a useful amount of exhaust). The measurement is optical: the system is sending a light beam across the roadway, roughly through the region where your car will be spewing exhaust, and a receiver on the far side of the road measures the attenuation of specific wavelengths of the beam by the components of your exhaust. The higher the concentration of criteria pollutants in your exhaust, the more certain wavelengths of the beam are attenuated.

Formal compliance testing of a vehicle or engine in an EPA or manufacturer test cell is done under exacting conditions to assure repeatability. The vehicle is “soaked” at a specified temperature for a specified time before the test, the test cell is kept at a specific temperature, the vehicle/engine is driven through specific sequences of speed/distance, the the exhaust is handled in a very specific manner (e.g. it’s passed through a “dilution tunnel” to simulate atmospheric mixing/chemistry before measurement), and so on. (if you want to know ALL the requirements for how to conduct a vehicle test, well, here ya go.) By comparison, as you might expect, a drive-by emissions test isn’t terribly precise because it doesn’t control for many of these factors. OTOH, it doesn’t need to be that precise, because when cars go bad WRT emissions, they tend to go really bad. The drive-by test isn’t looking for cars that are 5-10% over the limit, it’s looking for cars that are 5-10 times over the limit, and that kind of discrepancy is relatively easy to spot with a drive-by test.

The primary value of the drive-by test is that it reduces the inconvenience to most drivers by eliminating the need to go somewhere just for a smog check - unless the drive-by test specifically shows that your vehicle probably has an issue. If it does, then you are required to go to a mechanic shop for a more accurate emissions check. The drive-by test can also show model-by-model trends that can indicate a broader model-related problem that might justify an investigation/recall. For example, after the Volkwagen emissions scandal several years ago, a check of Colorado’s aggregated drive-by-emissions data clearly showed that certain VW models emitted a lot more on the road than they did in a test cell.

The limited extent of the testing (esp. the limitation to the Twin Cities metro area) was likely related to a failure of the region to meet one or more National Ambient Air Quality Standards (NAAQS). When this happens, the state is required to submit a State Implementation Plan (SIP) describing actions the state will take in order to bring local air quality into compliance. Vehicle emissions testing, restricted to just the metro area, was likely one aspect of their SIP. If they’ve eliminated this test program, it likely means that they’ve achieved NAAQS compliance and had reason to believe the program was no longer necessary.

“Environmentally friendly” depends on what aspect of the environment is being considered.

There are two different aspects of a car’s emissions that are kind of in tension with each other. One is the carbon footprint, which seems to be all the rage these days. The carbon footprint does matter - for your great-grandchildren and their descendants. But the other aspect is the immediately noxious emissions, most notably raw hydrocarbons, NOx, and particulate matter (PM), and these will adversely affect your health today. Engines/vehicles could be made more efficient (lower carbon footprint) in a heartbeat if we eliminated the restrictions on HC/NOx/PM emissions, but present-day air quality would quickly turn to shit. In fact, even with current emissions standards, air quality in many cities in the US (e.g. Los Angeles) is already a problem. Cars are vastly cleaner than they were 50 years ago, but there are also far more of them now, and we drive them much more than we used to.

NJ has eliminated safety testing, eliminated emissions testing on old vehicles, and only does the OBDII testing on newer vehicles (every 2 years), with brand new vehicles getting a 5 year exemption.

The inspection station I went to recently looked like a ghost town. 10 years ago, it was like the hottest bar in town, with a line out the door and around the block.

A mass spectrometer has a vacuum in the testing chamber, so that only the substance being tested is, well, tested. Lord knows what other gases are present with this method, which is probably why some flunk the “outside” test and pass the “inside” test.

For the roadside emissions check, the gases present are a blend of car exhaust and atmosphere. This is the same mix that’s analyzed in test cells during compliance/certification testing. In the test cell, part of the test procedure is assessing ambient/background levels for all criteria pollutants before starting the vehicle; the vehicle’s output is then the difference between the test measurement and the background. The roadside test equipment would necessarily need to do the same thing, i.e. assessing background levels in the time shortly before a vehicle passes by.

Probably the largest source of imprecision in roadside testing comes from mixing of exhaust with ambient air as the vehicle passes the test apparatus. In a test cell, the flow rate in the dilution tunnel is measured, so the dilution ratio is known. With roadside testing, they must assume some dilution ratio, and no doubt it ends up being way off every now and then. Probably not surprising that a few healthy cars fail the roadside check and then pass the garage check. There are probably also some dirty cars that pass the roadside check and receive no further scrutiny. Overall the benefits of roadside checking (in terms of convenience and cost savings) would appear to outweigh the detriment (letting a few dirty cars get by in any given registration year).

I think our last California emission test was around 1989, just before we moved from an urban-corridor area to coastal mountains, and later the Sierras, outside mandatory smog-check zones. Our vintage smokers never needed further work. Now we run a Sprinter diesel on a 2015 platform. It’s supposedly clean but we’ve never had to find out. If we innocently run it past any other states’ drive-by smog checks, should we expect a mailed notice?

Thanks, Machine Elf. What sort of sensors detect the presence of a car and the location of the tail pipe? If it is able to “see” the license plate, it has a general idea of the car’s orientation.

I’m sure it’s probably better. I was only wondering how it might measure emissions under different conditions, to the degree that that even matters. It might not. If they do it at on-ramp, as Machine Elf describes, we can assume they’re getting the most relevant measurement.

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Generally speaking, these systems are set up on highway on-ramps, so the cars are always passing the test point going in the same direction. I don’t know exactly how they detect passage of a vehicle, but this seems like a trivial challenge. Example, if the measurement beams are completely interrupted, then the system would known that a vehicle has passed by, and it needs to grab the required measurements.

The systems appear to place the measurement beams at a height above the pavement that’s a decent match for the height of the tailpipe on most light-duty passenger vehicles. Since they’re reading the license plate when each vehicle goes by, they’ll know (from state vehicle registration databases) what kind of vehicle is involved; if it’s something with a tailpipe configuration that prevents a valid reading, they’ll know it, and the test results can be invalidated.

Here are a couple of videos about Colorado’s system:

They have a person reading the plate. I thought it was license plate recognition software, hence the question. Thanks again, Machine Elf.

Perhaps you are boarderline, good enough to pass the general emission test but not good enough for this test, or even failing it. You may be running it with your engine still cold, thus failing it at the time you are testing. Or your exhaust is directed in a way that your car is not reading correctly, or your license plate is obscured.

In the UK an emissions test is one part of the annual safety check that is mandatory for cars over 3 years old. What has happened in those areas where emissions checks have been dropped? is it:
[li]There were no annual safety checks[/li][li]There were safety checks but those have been dropped too[/li][li]There are still mandatory safety checks[/li][/ol]

In most of the US, there are no safety checks ever.

In Oregon there are emissions tests in only a few urban areas, none at all in the rest of the state. No safety inspections.

My well maintained car, now 18 years old, has never been inspected by anyone. When registration tags are about to expire I renew them on the internet and new stickers arrive in the mail.

If a police officer happens to see a possible safety issue, or equipment not functioning correctly, they may pull a car over and issue a faulty equipment ticket. Fix the problem, show the police that it is repaired and no fine. Otherwise, most vehicles are never inspected at all.

Commercial vehicles are inspected and can be pulled over at anytime for a spot inspection, but not passenger vehicles.

The Arkansas department of motor vehicles stopped their safety checks several years ago.It was a racket for some garages. “Those brakes ain’t safe, M’am. You’ll need a break job to pass.” $$$ I don’t believe they ever did emissions testing.