How Did You Come Up With That Speedlimit

This brochure is designed to help answer the most frequently asked questions regarding how and why KDOT establishes speed limits.

It takes into consideration the different types of communities, roads, and traffic KDOT deals with every day. It also confronts most drivers’ conceptions as well as misconceptions about speed limits.

But, when you get right down to it, Kansas—like all other states—uses a widely practiced principle for establishing “safe and reasonable” speed limits. It’s called “speed zoning.”

Here’s how speed zoning works.

Every few years, as inconspicuously as possible, KDOT personnel go to various areas and use radar to randomly record the speed of as many as 100 vehicles—about 50 in each direction. It’s done over the course of one to two hours, during “off-peak, free-towing” traffic periods.

The reason it’s done this way—and not at rush hour or at other times when traffic is congested—is to provide normal conditions for determining the “truest” speed at each particular “zone” location.

As a result, it’s typically the local drivers in a community—those who “know” the road and drive it most often—who determine the speed limit.

“Don’t most people just naturally drive 5mph over the speed limit?”

That’s what a lot of people think. But the answer is no.

Speed zoning bears this out—by providing an unbiased cross-section sample of traffic. It also allows us to determine the speed of the majority of drivers. It’s what we call the 85th Percentile Speed—and, literally, 85% of the drivers will travel at this speed or lower.

According to research, the 85th Percentile Speed is also the safest speed.

“Why is the 85th Percentile Speed the safest speed?”

If you look at the lowest point on the curve in the graph above, it’s at the 85th Percentile Speed. It’s also the same point at which your chances of being in a crash are the lowest. Because that’s the area with the least speed variation.

And for every five miles per hour you drive— either over or under the 85th Percentile Speed— the greater your chances are of being involved in a crash.

“Why does Kansas set speed limits this way?”

All states and most local agencies use the 85th Percentile for establishing speed limits in accordance with the nationally accepted Manual on Uniform Trac Control Devices.

KDOT also conducts traffc studies every three to four years or whenever specifc safety concerns require more immediate attention in communities all across Kansas.

These studies take into account the results of widespread safety research and other factors such as:

• The total number of reported crashes.
• Types of on-street parking.
• Road and shoulder characteristics, including: turns and curves, dips and rises.
• The effect of speeds and speed variance.

They take local development into consideration, including such factors as: How many driveways are there? How many intersections? Are there traffic signals? How many signs? Is it mostly commercial through-traffc? Or local and residential? Any school zones? Or pedestrian crossings?

“Why can’t you just lower the speed limit?”

Studies show that just lowering a posted speed limit does not automatically equate to safer driving conditions.

If we arbitrarily lower the speed limit, a few drivers will slow down. Most others will continue driving at the speed they have been. The same happens if you arbitrarily raise the speed limit. In fact, all it does is create a wider variation of speeds which could result in a greater number of crashes.

To be effective, speed limits should:

• Help reduce the speed variation.
• Be realistic.
• Invite compliance.
• Serve as a clear reminder of what is a “reasonable and prudent” speed.
• Serve as an effective tool for law enforcement.
• Encourage drivers to travel at a speed where the risk of a crash is lowest.

Most people instinctively drive at speeds that are “reasonable and prudent.”

Enforcing the speed limit law gains the most public support when the posted speed limit reflects the speed the majority of motorists considers to be “reasonable and prudent. ”Think about it. How would you like it, if the personal preferences of others were imposed on you? Could you imagine four-way stops at every intersection? Or 25 mile-per-hour speed limits on rural highways?

“Isn’t slower safer?”

The answer is “no!” In fact, arbitrarily low speed limits can actually lead to more crashes.

Research shows simply posting a lower speed limit is no guarantee drivers will change their driving habits.

It’s been our experience that most public outcries for “lowering the speed limit” usually occur after some personal or high profile incident or some new development stirs up the public’s concern and emotions.

Law enforcement can’t be “out on the roads” all the time. If lowering the speed limit makes most of the drivers “violators,” and they ticket as many as they can catch, it just creates a lot of ill will. And their town gets labeled as a “speed trap.” In actuality, the intent of having any speed limit at all is to make driving in Kansas safer for everyone.

“So what speed is reasonable and prudent?”

Every location is unique.

What’s “reasonable and prudent” on a highway that bypasses a city is different than where a highway goes right through the heart of a city.

A two-lane highway in a rural setting is different than a four-lane highway lined with homes, businesses and schools.

In each case, however, the overriding concern of the Kansas Department of Transportation is safety.

Ultimately it’s up to everyone who drives on our Kansas roads to truly make our highways safe.

Obeying the posted speed limit is just one aspect of safe driving.

It’s important also to stay alert to road, weather and driving conditions, buckle up, and always use courtesy and common sense.

• Don’t tailgate.
• Use your turn signals.
• Stay out of the left lane unless you are passing.
• Your horn is for warning people of danger, not to make them go faster.
• If you wouldn’t “jump the line” in the grocery store or the movie theater, don’t do it when lanes are merging.
• The only hand gesture you should be making is a friendly wave.
• Slow down in construction zones.
• Don’t drink and drive.
• Pay attention to the road and the traffc, not your passengers, your radio, your make-up or your cell phone.

Click on this link for the PDF file.