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.