Episode 11 – Traffic Calming

Topics: Traffic Calming, Chicanes, Speed Humps, Speed Bumps, Safety, Statistical Analysis

Talking Traffic Episode 11 – Traffic Calming

Hello, and welcome to the eleventh episode of the geeky transportation educational podcast entitled Talking Traffic. My Name is Bill Ruhsam, and I am the creator of this audio production. Today is Tuesday, February fifth, 2008. I would have liked to have a groundhog day episode, but my schedule last week was hectic enough to keep me from having the time to put up a worthwhile piece of educational literature before today. As I’d rather cut off my pinkies then produce a crappy podcast (and therefore be unable to type the letters Q, A, Z, P, Colon, Question Mark, and SHIFT), I decided to hold my normal schedule. Well, almost my normal schedule. If you read my personal blog, you would have seen some dissertations around the end of last year concerning priorities and personal scheduling. Something about how podcasting takes a third slot behind some other life goals. The upshot was, when the going gets crazy, you need to set some concrete mileposts to let you know when you’re leaving your assigned path. Those mileposts keep you on track and paying attention.

Kind of like the topic of today’s Talking traffic! I hope I didn’t stretch the metaphor too much, but we’re going to be talking about Traffic Calming, the diabolical plan by community organizations to make your neighborhoods safer and prettier! We must stop them now!

Seriously, Traffic Calming means different things to different people, but fundamentally, it is an approach to design, construction, and maintenance to encourage slower, more attentive driving. Slower speeds and more attention mean safer roads because collisions are either averted, or the ones that occur happen with lesser severity. There are a few studies that tell of an exponential curve relating higher speeds to greater chances of pedestrian and bicycle fatalities. Unfortunately, these studies haven’t been comprehensive. However, common sense seems to indicate that your chance of injury or death goes up as the speed with which an object strikes you climbs. Debates are ongoing over whether residential speeds of 25 mph or 20 or even 10 mph are safer than faster speeds, but I can state with near certainty that lower speeds are safer than higher speeds.

BUT! (Huge “but” here) What do we mean by safer? Is it more likely that faster drivers get into wrecks? Is it more likely that slower drivers DON’T get into wrecks? We don’t know. As my psychologically educated friends like to say, correlation does not imply causality. While a high percentage of fatal collisions involve excessive speed, it does not necessarily imply that excessive speeding causes fatalities. This is like taking a sample of people at your mall, noting that the ones eating at McDonalds are wearing red and thus concluding that wearing red makes you eat at McDonalds. If there’s any tiny bit of knowledge you get from this episode, this might be the one thing: Statistics and Studies quoted in news articles and reports can be misleading at best, and downright lies at worst. It’s easy to take the complicated conclusions of a statistical analysis and bend them around to say what you want such as all of the political polling going on right now. You may not realize, but the percentage error they quote at the end of those polls is very different, statistically, then how they imply it. For example, they may state that Senator Bullworth has a job approval rating of 40% with an error range of 7%, and you might think that this means they had some wishy washy portion of interviewed citizens who couldn’t commit? No! You’d be wrong. The error rate quoted in the polls has to do entirely with the sample size and the confidence interval of your analysis. The larger the sample and the tighter the confidence interval, the less error you will have. But, if you use the same sample and the same confidence for all of your political polls, your error range will be the same now matter who you ask, or where they live. Statistics are tricky, and political analysts will bend them to make them appear to say things that they want. Beware of statistics!

All right. End of rant. We were talking about traffic calming. As I mentioned, traffic calming is intended to make drivers more attentive and to lessen the chances of a fatal or severe collision. Traffic calming is usually used in the context of the interface between motor vehicles and pedestrians or bicycles. The Institute of Transportation Engineers, of which I am a member, published a report in 1999 where they defined Traffic Calming as “…the combination of mainly physical measures that reduce the negative effects of motor vehicle use, alter driver behavior and improve conditions for non-motorized street users.” Basically what I said before, just much more professional-sounding. ITE is an excellent professional organization, and it devotes itself to furthering the state of practice for traffic engineering. I’ll link in the show notes to their traffic calming website.

Traffic calming comes in many different flavors and is used in various forms throughout the United States and the rest of the world. A quick list of some things that are considered traffic calming devices is:

  • Roundabouts! (remember episode 4?)
  • Speed humps, speed bumps, speed tables, etc. They are all the same device, with different designs.
  • Chicanes (more on that is a second)
  • Raised intersections or tabled intersections
  • Intersection curb bumpouts and center medians
  • Narrowing travel lanes
  • No Traffic Control

This list can also be summed up in another way: Traffic calming measures are either Vertical Deflections of vehicles , Horizontal Deflections, or psychological pressures. Vertical deflections are things such as speed humps, speed tables, and raised intersections. Examples of Horizontal deflections are Roundabouts and chicanes, while psychological measures include the narrowing of travel lanes and curb bumpouts (also called chokers).

I will link in the notes to examples of each of these listed traffic calming measures, but I think I can regale you with some descriptions, and how effective and/or controversial they are.

First off, Speed Humps. Speed humps are raised sections of pavement on low-volume residential roadways which deflect vehicles in a vertical direction. They are typically 12 feet long as measured along the roadway and have a curved ramp up and down from the top of the hump. The precise design differs from location to location, but that is a good rule of thumb. Speed humps are designed to slow vehicles by making it uncomfortable to drive at a higher speed than what is posted. Speed Bumps, on the other hand, are not used on streets and highways. They are usually only installed by businesses and property owners without the same types of public liability concerns as Public Works departments or DOT’s. Speed bumps are short, and sharp, and work best at low speeds. At High Speeds, a car’s suspension will absorb most of the shock, but therein lies the problem with speed bumps; if you are hitting one at high speed, the likelihood of damage to your suspension or undercarriage is high, with the possibility of control loss, leading to collisions. Speed bumps are not encouraged. Speed Tables, on the other hand, are only a form of larger speed Hump. Where a speed hump is generally 12 feet long, speed tables are specified as being 22 feet or so from end to end. This means that a vehicle’s wheel base will be entirely on top of the speed table while driving over it.

Speed humps are growing in popularity around the country, but they also have a significant number of detractors. There are communities that absolutely forbid the construction of speed humps. No way, no how. There are other communities that insist on a supermajority of the citizens living near the speed hump to approve. Some will install them after a request is made and an engineering analysis is conducted, indicating a need. These, of course, are individual policy decisions which will differ depending where you live. Remember, that the purpose of a speed hump or table is to control the speed of a vehicle.

Raised intersections, or tabled intersections, are a form of speed table, but with an additional pedestrian component thrown in. They take an entire intersection and raise up the pavement level to just beneath the curb. This preserves a small lip for visually impaired persons to find the edge, and makes the entire intersection much more pedestrian friendly.

The second main type of traffic calming measure involves horizontal deflections. These can be accomplished by using a series or reverse curves in the roadway, which forces the driver to swing back and forth, or by installing side-islands that stick out into the existing roadway in a fashion that vehicles may not move in a straight line. They must navigate around these barriers in order to traverse the roadway. These types of devices are known as chicanes, whether they are installed deliberately during initial construction, or added on afterwards. Another type of horizontal-deflection traffic calming device is the modern roundabout. (Not a rotary or traffic circle. Those are different. For the details, listen to Episode 4.) A roundabout is an intersection treatment that ensures slower speeds and lesser-severity crashes. Roundabouts force drivers to travel in a circle, yielding to vehicles already inside the roundabout, then exiting to the right when they reach their desired exit. The intersection does not need any additional traffic control such as a signal or a stop sign aside from the yield signs placed for entering vehicles. Roundabouts are not the answer to all problems, but they are excellent treatments for mid-sized intersections, plus they are operationally efficient. Within certain parameters, you can get more vehicles through a roundabout than through a signal or stop sign.

Psychological factors are also employed to calm traffic. If you’re driving a wide open road, you’re likely to go faster. Narrow that roadway, and you’re likely to go slower. Thus we can narrow the traffic lanes, or the shoulders; bring the curb line right up to the edge of travel, which also assists pedestrians when they are making crossing maneuvers. This is often called a curb bumpout. Or we can add a center island with landscaping which also restricts the width of the roadway. Trees, planters, benches, and other accoutrements along the curb will also contribute to a calming effect, as will textured pavement such as brick pavers. These types of treatments fit in well in a downtown business district, or a multi-use community.

Of course, you could go the direction of the town of Drachten, in the Netherlands. Their approach to downtown business traffic control is that less is more. They have deliberately removed all of the traffic control devices such as signs and signals from the downtown area. They have left in place the striping, but after that it’s every person, vehicle, or bicycle for themselves. The theory is that if you throw everyone into the same shared space with no direct assignment of priority, then they are all forced to pay attention and to take or yield the right of way as appropriate at that time. To date, that particular experiment seems to be working well, with a significant drop in fatalities and traffic jams.

The measures I’ve just described do not comprise a comprehensive list of traffic calming. There are plenty of others. The important thing to remember is that Traffic Calming should not be installed willy nilly. You may be buying trouble for other parts of town, or even for yourself if someone decides to sue. Proper engineering investigation is always appropriate when installing physical objects in the public right of way.

And, don’t forget that traffic calming may be inappropriate. Sometimes traffic calming isn’t necessary despite a perception that it is. An example would be a residential neighborhood with a perceived speeding problem. Just a few fast drives can make it seem like there’s a huge issue, when maybe the average is not any higher than the posted speed. Actually, average speed isn’t used frequently when studying speed characteristics. We use a term called the 85th percentile speed, which is the speed that 85% of the vehicles are traveling at or below. More on that in a later episode talking about speeding and speed analysis. Suffice to say that the 85th percentile is the recognized standard. So, if the 85th percentile speed is 26 mph in a 25 mph zone, there is no speeding problem. Perhaps your neighborhood needs to lean on the crazy driver who’s making the road a drag strip? If it’s not a local, then police enforcement will be necessary. Perhaps necessary even if the person *is* a local. The point is, that sometimes traffic calming measures are not the solution.

If you take anything away from this episode, aside from my dire warning about statistics, remember what traffic calming is, and what it isn’t. To repeat the quote from ITE, traffic calming is “…the combination of mainly physical measures that reduce the negative effects of motor vehicle use, alter driver behavior and improve conditions for non-motorized street users.” Traffic calming is roadway improvement to make drivers pay attention and be safe. What traffic calming is *not* is regulatory devices or enforcement. By its nature, traffic calming is designed to be self-enforcing.

I’ve had some feedback on the podcast in the past few weeks. I want to thank all of the people whom I’m not already closely associated with for sending me complimentary emails. I wouldn’t do this if I didn’t enjoy it, but it certainly helps to know that there are people out there listening and getting something out of the information. One of the reasons behind this podcast is that there are plenty of intelligent, well-informed people who don’t know the basics of transportation infrastructure. I don’t blame them. If I weren’t a traffic engineer, I wouldn’t have much clue about this stuff either. It’s like air, it’s all around, but we don’t’ pay attention to it. This can be problematic if you want to talk to the professionals in your community, such as your local public works department or a designer working on a project. If you don’t have the vocabulary, it’s hard for us, the professionals to truly understand what you are getting at, and it’s hard for you, the concerned citizen, to ensure that we have understood your point. I try to make a special effort when I’m talking with non-transportation people, but I am still guilty of speaking jargon, because it’s my job, and I do it all the time. Habits can be hard to break. This podcast will try to get everyone to meet halfway.

This eleventh episode of talking traffic is released under a creative commons 3.0 attribution, non commercial, no derivatives license, which means you can copy and distribute it, but don’t change it or sell it, and make sure you credit me and Talkingtraffic.org. The theme music is by Five Star Fall off of magnatune.com, and I thank John Luton, Voodoo Zebra, Richard Drdul and Jo McEntire for their kind permission to use some flickr images for showing concrete examples of traffic calming. Their flickr sites are linked in the show notes. If a picture is worth a thousand words, I think Flickr has the entire library of congress by now.

Thanks for listening. If you have any comments, or want to start a petition to have a tiger pit installed in the road in front of your house, you can send an email to Bill @ Talkingtraffic.org, or just leave a comment on the show notes.

Next week we’ll talk about feeling the need for speed and how the transportation community deals with it.

Theme Music: Five Star Fall, Mercurial Girl, Magnatune.com
ITE Traffic Calming Information

Traffic Calming Examples:

Image of a roadway chicane intended as traffic calming.


Photo credits go to Richard Drdul. This photo is licensed as a Creative Commons 2.0 Attribution, Share-alike, per the original photographer.

Image of a speed hump intended as a traffic calming device.

Speed Hump:

Photo credits go to Voodoo Zeebra. This photo is licensed as Creative Commons 2.0 Attribution Non-Commercial per the original photographer.


Diagram of a modern Roundabout

Image of a center median narrowing as a traffic calming device.

Center Median narrowing:

Photo credits go to John Luton, Executive Director, Capital Bike and Walk, Victoria, BC

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