SAN ANTONIO -

The Texas Department of Transportation confirms the overturned 18-wheeler with about 3,500 gallons of sodium hydroxide was on one of San Antonio's designated routes for hazardous cargo trucks.

The accident occurred late Tuesday afternoon at I-37 and Loop 410, but San Antonio police reports the chemical leaks were minimal.

It was in stark contrast to the 1999 calamity involving an 18-wheeler carrying hydrochloric acid in the heart of downtown San Antonio.

The rig flipped over on the exit from I-35 South to I-10 West.

"Homes, schools, evacuating, businesses. I remember that incident like it was yesterday," said Laura Lopez, TxDOT spokeswoman.

She said that area was paralyzed for 24 hours, and affected other parts of the city as well.

Lopez said the incident led to the creation of hazmat routes in San Antonio.

"The downtown area is the area that they are to avoid," Lopez said.

An SAPD spokeswoman said a city ordinance declares the upper and lower freeways through the central business district, off limits to hazardous cargo.

She said traffic officers rarely have to stop hazmat trucks in that area, but if they do it is a city violation.

Lopez said since 1999, "We haven't seen too many of them, however, every once in a while an incident happens."

She said even then, they usually are on a hazmat route.

To see a map of Hazmat routes in Bexar County, click here (PDF).