The Texas Transportation Code states that “an operator may not drive at a speed greater than is reasonable and prudent under the circumstances then existing”. Therefore, regardless of the posted speed limit, every driver has a duty to drive at a reasonable and safe speed for the conditions at that time.
There are two kinds of maximum speed limits. All roadways in Texas have a default speed limit called the prima facie limit based on the type of roadway. This applies to residential streets (30 mph), alleys (15 mph), interstates (70 mph), etc. If the controlling jurisdiction believes that the prima facie limit is unreasonable or unsafe due to conditions on a specific roadway, the speed limit can be changed. In Grapevine, this has to be substantiated by an engineering and traffic investigation, adopted by the City Council, and then posted on speed limit signs.
An engineering and traffic investigation 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 to be 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.