It sounds like it was mostly designed to be enforced against lawn care companies.
Meanwhile our city's traffic lights are often 180 degrees out of phase (driving the limit from one light to the next gets you there as the light changes to red). Buses and semi-trucks are allowed to idle here too (not so in NYC).
They want to push new law enforcement on the civilians rather than actually address the sources of pollution. When Houston does dip down below "acceptable" air standards (dipping down for 60 minutes in one day counts as a full day) it can be seen to be coming from the petrochemical industry in time readings of the pollution.
This whole "Houston has bad air" thing came up as a political issue because George W. Bush was a Texas Governor running for President. We lost our status of worst air to LA again. The standards were changed and I suspect that they were changed so that Houston would get that distinction in a presidental election year.
Mayor Leepy Period Brown refused to counter the Rat charges about Houston's air until after the 2000 election. Now Houston faces a PR nightmare.
Houston's bad at times but the landscape is flat and we are at sea level. Things blow over (as they do at Galveston with that Gulf wind).
When the West Coast gets heavy forest fires, we can get some smokey skies in Houston. We all feel the effects of other regions air.