Here is the info for Rapidpass -
http://www.deq.virginia.gov/Programs/AirCheckVirginia/ForMotoristsVehicleOwners/RAPIDPASSVirginia.aspx
I know quite a bit about the program, it has been in the works for like 10 years, lots of prototypes and testing went into this.
Finally rolled out in the last 2 years as I recall.
So here is the scoop as I understand it.
1. If your car has recently failed an Emission Test you will not qualify for the Rapidpass program, they want your car to visit an Emission Test station.
2. I know it can take a while for the cars data to show up on the Rapidpass site. You can visit the site and see if your car was checked, if it passed and pay the Inspection fee if they show your vehicle was tested and was clean.
3. I went though a site back in October as I recall, my car was not due for Emission Inspection unit this month, March. I checked the site a few times after I drove through back in October and never found anything, forgot about it. In February I was mailed a Rapidpass letter than the car had been tested and was clean and I could pay the fee and then I could renew my tags. I waited to pay the fee in early March because it was unclear if I paid the fee and renewed my tags in February would I loose a month.
4. Unclear how long it takes to upload the results, I would expect that evening the data from the machines is captured but it may take a few days/week to merge everything via the DMV license plate database and then upload the info to the Rapidpass site??
5. There are gotchas. Wind, rain and weather conditions can screw up results. Other things like the camera not getting a clear shot of the license plate or some failure or problem with the computers running the machine may make the sample invalid.
6. If the car is registered to a different physical address, you may not get a letter, but you will ONLY get notified by mail if the car has not failed an Emission Test and the RapidPass test was sucessful AND the exhaust was deemed clean enough to pass.
Like anything there can and will be problems, but overall the system does work. RapidPass does not deploy the equipment during bad weather or windy days. Even if the equipment is deployed the weather may change during the day and only some of the samples will be accurate.
The sad but true part of the RapidPass system is your car can have the SES/CEL/MIL lit on the dashboard and/or have too many Emission Readiness Monitors not Ready and/or have emission components altered and/or removed and as long as the drive by test is clean enough for the vehicle year, you can PASS.
Do not think you can trick the system and turn off the engine to beat the test, there is an IR thermometer that is part of the system that can and will detect if the engine is fully warmed up and that the exhaust is at the correct temperature and there is exhaust actually exiting the tailpipe(s).
It can be a convenience because there is a list of where the testing sites will be deployed so you can arrange to drive through them if they are nearby, but it is unclear how long it takes for the results to be available. Kind of a crap shoot at this point, but worth trying the RapidPass testing if there are sites near where you travel.