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  • Ren Gudino

How are Speed Limits Determined?

On a recent road trip, I couldn’t help but notice the speed limit ranging from 45 to 75 within one ten minute stretch. Maybe you’ve experienced that before: traveling along an interstate, you exit to take a highway, and suddenly you’re crawling along in the middle of a quaint town that disappears in five minutes and suddenly you’re directed to be back at top speed. Why does this happen and how are these speed limits determined?

First, there are statutory speed limits. These are the limits set by state legislature and apply even when there isn’t a sign posted. These are examples of Statutory speed limits:

  • 25 mph in residential or school districts,

  • 55 mph on rural highways, and

  • 70 mph on rural Interstate highways.

Then there’s a posted speed limit. These are posted on signs and can be the same OR can differ from a statutory speed limit, if the city, state, or county saw a need to adjust it. Whenever there’s an adjustment to the statutory speed limit, an engineered speed study has taken place. We’ll explain what that is and how it’s determined later in this post.

Finally, there are “special conditions” speed limits, for school zones, work zones, and instances with variable speeds. School zones range from 15-25mph during school hours in areas to and from schools. A work zone speed limit is used to manage traffic whenever there may be workers or a work zone on a road. Decreased speeds make for a safer work environment for the road workers and also keep traffic moving smoothly. A variable speed limit is a temporary sign set up when the variable is 10 mph slower than the usually set limit. These are put in place for variable conditions, such as inclement weather, work zones, incidents, or traffic.

Back to our main question: how are these speeds determined? When designing a road, engineers consider the speed of traffic using a design speed: a conceptualized speed that determines how the road will be built and what will be included (if it’s residential, in a business area, if there is on-street parking, etc). Once the road is build, a speed study takes place, monitoring the speeds that drivers typically travel on that road. Afterward, most engineers follow what is called the “85th percentile speed” rule or use a program called USLIMIT2. Once traffic has been monitored on a road, engineers calculate what is 85% of the average traveler's speed and use that to determine what should be the limit. This way, people who would have a tendency to speed regardless are (hopefully) encouraged to go a little slower than usual and their speeding numbers won’t skew the set speed limit. There are problems with this, of course. The speed limit can also be determined using the program called USLIMIT2, which also uses a speed study, but will be covered in more detail in the future.

Both the 85% percentile rule and USLIMIT2 program have their pros and cons but we’ll get into that another time. For now, let’s say that whenever you’re driving along and are wondering, “Why on earth are these speeds changing so much?” the answer is likely because other drivers naturally traveled at an average of those speeds, and the limit was adjusted and set for safety.

Speaking of speed limits, did you know that we print those signs? That’s right! Our folks here at Interstate Signways printed, laminated, and sent off those signs to your state’s Department of Transportation. With every sign, we’re wishing you safe travels along your way!

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