The Texas Department of Transportation sets the speed limit on Main Street. They will periodically conduct an engineering and traffic investigation, which typically involves a survey of actual motorist speeds during free-flow conditions. The survey provides the 85th percentile speed, which is the speed at or below which 85% of the motorists are traveling. The speed limit is typically then set within 5 mph of the 85th percentile speed. The speed limit can be adjusted slightly to account for sight distance restrictions, accident history, presence of driveways, and other factors. Setting the speed limit close to the 85th percentile speed ensures that the speed limit reflects the speed that the majority of drivers consider being reasonable and prudent based on the conditions.
Residents often request that speed limits be lowered with the expectation that this will lower traffic speeds. However, studies have shown that most people drive at the speed they are comfortable with for the given conditions regardless of the posted speed limit. There is little or no significant change in speeds following the posting of a revised speed limit. This is true whether the speed limit is increased or decreased. Also, safety is not improved by establishing unreasonably low-speed limits, since this only encourages more variation in vehicle speeds, leading to more conflicts.