Speeding Limits & Car Crashes: Exploring The Intersection

Speed's role in car crashes

Streets with low or mid-range posted speed limits often get written off as “safer”, or lower risk of crashes. But the data does not seem to back up this common notion. In fact, this idea can desensitize us to distracted driving on streets with mid range speed limits to a point that actually makes them considerably less safe than a highway. On the highway, most of us would interject if a driver was blatantly on their phone, changing a song or typing a text. However, at lower speeds there is less of an urgency behind preventing distracted driving.

When looking at factors that affect the severity of a car crash, the speed at which the drivers are going is at the top of the list. However, what if the scope of interest is modified slightly? What if instead of looking at how the speed of the drivers affects the severity of a crash, we look at how the speed of the drivers affects the likelihood of a crash in the first place? Well, this issue with this scope of view is that the likelihood of a crash at a given speed may differ at different posted speed limits. For example, it would be within reason to say that someone driving at 25 mph is less likely to get into a car crash than someone driving at 60 mph— where the posted speed limit is 25mph. But what if we are looking at a street with a posted speed limit of 60 mph instead of 25 mph. Would the former still be a reasonable assumption? While the answer requires a lot of data and in depth analysis, the question itself guides us to the conclusion that using the posted speed limits themselves may be a more consistent and convenient indicator of the likelihood of car crashes.

The posted speed limits offer us a soft estimation of the speed at which drivers are going. As every driver knows, this is by no means an average or a maximum, but it does give us some idea of how fast drivers are driving. Additionally, the posted speed limit also can give an idea of the terrain of a road. A more windy road for example tends to have a lower speed limit as a driver must be prepared for the many twists and turns, while a highway can have a higher speed limit as it is a fairly smooth terrain. 

What's the right posted speed limit to use?

So, at what posted speed limits should we expect the lowest percent of crashes in Texas? Streets with posted speed limits less than 30 mph have significantly less crashes than any other posted speed limit (with the exception of 80-85 mph due to the fact that these posted limits experience less traffic than any other speed limit). Since drivers tend to drive slower on these roads as opposed to others with higher posted limits, they are able to have more time to react to a potentially unsafe situation. That is not to say that distracted or otherwise unsafe drivers do not exist in these areas— that is far from the case. Almost 19,000 crashes occurred in areas with posted speed limits less than 30 mph over the years 2017-2019 in Texas. However, in comparison to total accidents in that same range, this accounts for less than 6%.

The posted speed limits with the highest percent of crashes fall in the range of 30mph-45mph. This range of posted limits is home to almost 62% of crashes in Texas. Although this range does not contain top speeds, this does not mean that these crashes did not involve cars driving at those top speeds and well above the posted limit. The bottom line is that these roads were not meant for drivers going at top speeds, especially in comparison to roads with limits greater than 45 mph which are more likely to be able to safely handle faster drivers.

So, by evaluating the likelihood of getting into a crash at various speed limits, we can conclude the 30 mph - 45mph zones may be the most dangerous and may require the most caution.