The Texas State Transportation code § 545.352 defines lawful for speeds for various types of roads.  Inside a city or town, 30 miles per hour is assumed to be a safe and lawful speed, unless you are driving in an alleyway. Then, the speed limit is 25 miles per hour. Once you get onto a numbered highway and away from urban and residential areas, the speed limit jumps to 70 miles per hour. On an unnumbered highway outside of an urban district, the speed limit is usually 60 miles per hour. On some parts of Interstates 10 and 20, the speed limit is set even higher, at 80 miles per hour.

However, as you've probably noticed by now, there are many instances in which Texas speed limits don't match these "default" limits. The next section of the Transportation Code gives municipalities the right to set different speed limits based on local conditions. Sometimes, it seems to drivers that these speed limits are set arbitrarily. However, in reality the law requires that traffic and engineering studies be performed to determine a speed that is not only safe but comfortable for the majority of drivers. Generally, engineers measure how fast drivers drive on the road in question, and set the speed limit based on the speed that 85 percent of the drivers are moving at or below. Texas speed limits may be set lower than this 85th percentile limit if there are special hazards such as curves and hills, or high numbers of accidents on the stretch of road in question. So, the next time you see a Texas speed limit sign, remember that a lot of thought has gone into choosing a reasonable speed limit.