Topics: Rumble Strips


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• Theme Music: Five Star Fall, Mercurial Girl, Magnatune.com

Hello and welcome to another edition of Talking Traffic. My name is Bill Ruhsam and I host this podcast and its sister website, talking traffic dot org. Today is Monday, June 13, 2011. This is episode 38 of Talking traffic and today I’ll be talking about those washboards of the road, Rumble Strips. Sometimes known as shoulder texturing, noise strips, rumble bars, or “Driving by Braile”. I’ll talk about what a rumble strip is and what it is for, plus their implications for safe driving

You will typically find a rumble strip along the edge of a high-speed roadway. These strips are designed to alert you when your tires roll over them by making a strong a vibration and noise. This tells you to that you are leaving your lane, on the way to a meeting with a tree, or possibly another car in a head-on wreck.

Rumble strips can be found on both your right side, on the edge of the road if you drive in the US, and on the left side, on the centerline. That second kind, on the left, is usually called a Centerline Rumble Strip, to distinguish it from Edgeline Rumble Strips.

These types of rumble strips are also referred to by pedantic engineers such as myself as Longitudinal rumble strips. In other words they are placed on the pavement *along* the roadway, rather than across it. You may also be familiar with rumble strips that *cross* the roadway. These are called transverse rumble strips and are used to alert you that you are approaching some sort of unusual location. Typically, I’ve seen them placed in rural areas before intersections that have had a high number of run-through-the-stop sign collisions. The rumble strip is to “wake you up” or just to alert your hazed and road-numbed brain that you need to stop.

Rumble strips are constructed in several different ways, depending on when they placed on the road. If they’re built when the pavement is built, then they can be cut, rolled, or pressed into the pavement as a part of the pavement laying process. On existing asphalt roads, they can be cut into the pavement. You can also use raised pavement markers or striping to make a rumble strip, although those are not as durable as ones that are built or cut into the pavement.

Do rumble strips work? Yes they do! Of course, you have to define what “work” means in order to make that statement.

Rumble strips are effective at preventing run off the road collisions if you drift to the right or head-on collisions if you drift to the left. However, they are dependent on having an adequate room to recover once that rumbling vibratory sound starts you back to awareness. If you are running off the road and immediately after you hit the rumble strip you’re on a soft shoulder or heading down an embankment into the weeds, that is not helpful. A wide safe shoulder is necessary to allow you time to correct and get back onto the roadway.

One dilemma that arises with rumble strips, however, is the concept of “migration”. “Migration” is the concept that rumble strips don’t stop the crash that would have been caused by an impaired or inattentive driver, rather it merely *migrates* it down the road to a point where the driver might cause a different sort of crash that could involve other vehicles. So instead of preventing the single-vehicle rollover or tree-strike, the rumble strip might cause a multi-vehicle collision of some other nature. There have been some studies that *suggest* this occurs, but to my knowledge there aren’t any that have concluded that it *does* to any degree of scientific certainty. It’s a difficult problem to address. How can you tell if that drunk driver who hit the family of five would have instead run off the road five miles back if the rumble striop hadn’t prevented it? the only way to really know is through statistical means, but the noise in the accident database is high enough that it would be difficult to detect a signal that small.

Lets see. What else is there about rumble strips! Oh, back in Texas, I had proposed that we put transverse rumble strips that played a song. Something like the William Tell Overture. I’m sure I wasn’t the first person to think of this and guess what! The Japanese have done it! They cut transverse strips into concrete on several roads to produce music.

Lastly, let’s talk about bikes. There’s all sorts of controversy about bikes and rumble strips. There is, in my opinion, a disconnect between the engineering community and the biking community on what makes a “bike friendly rumble strip”. The current design is to put a rumble strip just outside the striped edgeline of a roadway with a bike-friendly shoulder. A bike friendly shoulder is one that is wide enough for a cyclist to use it as their own roadway. The rumble strip is dashed so that there are gaps between the rumbles where a cyclist can move on and off the shoulder without running into the rumble strip itself. So far so good.

The problem lies in a fundamental problem with having “bicycle friendly shoulders” in the first place: debris. Rocks and sticks and trash and junk and tire-cutting metal tend to accumulate on the sides of roads. This detritus will only be removed by a cleaning operation of some sort, or by the wind-passage of vehicles. That’s why you don’t tend to see a lot of trash in the travel lanes of roads, but you *will* on the shoulders. These shoulders, of course, where cyclists are supposed to ride. A cyclist who is using the shoulder will stick closer to the travel lane than the grass because that is where less debris is. However, the rumble strips force the cyclist closer to the grass and more into the accumulated debris. That causes the cyclist to abandon the whole “bicycle friendly shoulder” concept and ride in the lane.

So that’s an issue. A solution for the *cyclist* would be to put the rumble strip farther out in the shoulder, but that isn’t a solution for the automobile. It’s a problem that doesn’t have a “right” solution. I can talk more about bike advocacy later.

So what did we learn? Rumble strips help you not leave your lane, and they work! Good job, rumble strips.

Thanks for listening to Talking Traffic. If you have any comments on this episode, or the podcast in general, feel free to post them at the website or send an email to bill at talking traffic.org.

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Until next week, have a great week.