Police said the 65-year-old man was standing near his truck, helping close off Loop 410 over Marbach due to a separate accident just before 3 a.m.
Police said the driver of a compact SUV who was southbound failed to switch lanes safely and hit the man.
The driver remained on scene and showed no signs of intoxication, but the worker was taken to University Hospital in critical condition. His name has not been released.
HERO stands for Highway Emergency Response Operator program. The free program provides a number of helpful services to local drivers.
HEROs help clear roads and restore normal traffic flow after crashes and they can help fix flat tires, provide gas or water, jump start batteries and perform minor vehicle repairs.
It has been operating in the local area for about two years. Workers are available 24 / 7 and they cover about 239 miles of local roadways.
The program is funded by TxDOT, Bexar County, the City of San Antonio and the Alamo Area Metropolitan Planning Organization.
The number to call to reach a HERO is: 210-732-HERO or their website.