Authorities may have been more concerned with explosions of stored chemicals than the gas leak itself. When I moved away from SE Houston in 1980 they were *still* devoting a full day in school classes to the
Texas City explosion nearly 40 years earlier of the same material.
(they also still spent a day covering the Galveston Hurricane 80 years earlier - some wounds cut deep)
There are roads leaving Bryan but 70,000 people is a lot of cars, and there is no mass transit. Texas A&M is a large school nearby and I bet a lot of students have no transport of their own.
Evacuating Texas A&M when campus is full during the school year is probably quite a problem. Right now the local public school districts can't plan to use their buses since their drivers are not available. Not to mention fuel stocks for the buses.