Texas has the ability to change speed limits by County. Heading South, the county prior to Webb County has a speed limit of 75 for cars and 70 for Trucks (different signs). At mile marker 38, there is a speed limit change from 75 to 70 but if you are not paying close attention, you could assume that the 70 mph speed limit applies to trucks. There is a warning sign that there will be a speed limit change, but they are fairly close together and could be missed.