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This is from a Texas DPS website:

November 1998

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DPS Begins Remote Emissions Testing

The DPS began testing vehicles commuting into Dallas, Tarrant and Harris counties for excess pollutants as they pass by roadside detectors in those counties. The "Gross Polluter Identification and On-Road Testing" component of the Texas Motorist’s Choice Program started in early October.

"This required component enhances the tailpipe emissions testing program already in place in Dallas, Tarrant, Harris and El Paso counties," said Major Ricky D. Smith.

The Gross Polluter Identification and On-Road Testing component is intended to provide cleaner air for Texas citizens and reduce pollution in areas where smog levels exceed federal health standards.

Instead of expanding station-based tailpipe emissions testing to the nine other ozone non-attainment counties (Denton, Collin, Brazoria, Chambers, Fort Bend, Galveston, Liberty, Montgomery and Waller), Texas will use on-road testing to focus on vehicles from those counties that commute to Dallas, Tarrant and Harris counties and are grossly violating air quality standards.

Gross Polluter Identification and On-Road Testing is a non-invasive technique that samples the exhaust of vehicles as they pass on-road detectors. The devices are located in and connected to vans that are strategically parked along roadways. While vehicle owners may be aware of the presence of devices, they are not required to stop or change their driving habits for the test.

Austin-based Tracor Aerospace, Inc. designed the system using tunable diode laser technology that samples the exhaust gases of a vehicle as it travels by sampling devices.

The registered owner of a vehicle identified as a ‘gross polluter’ will receive a notice that will instruct them to present the vehicle for a verification test at one of more than 2,300 certified emissions testing facilities in the non-attainment counties.

After the verification test, the vehicle owner may be required to seek necessary repairs to bring the vehicle into program compliance, and present the vehicle for a re-test to obtain an emissions inspection certificate. Failure to comply with the requirements of the notice is a criminal offense.

--Tela Goodwin Mange

The place I have seen them is the eastbound on-ramp at Trail Lake Dr and I20.