Modern Roundabout Truck Aprons 4

[TrafficDesigner]

I am researching different types of truck aprons for a roundabout design group that I am a member of.

I am interested in aprons used in the construction of Modern Roundabouts and I am curious as to what experiences anyone has had and if they prefer one type over another.

Specifically, what slope have you used, did you drain it inward or outward, apron width and material type.

If possible, any lessons learned after construction was completed. (good or bad)

I have read Ourston's guide, and FHWA guide along with some others, but I am more curious to learn from what people have actually done and the results of their work.

Thanks in advance for any advice or comments, in return I'll share what I know about the design of Modern Roundabouts.

 

[civilperson]

Roundabouts are traffic calming devices which are despised by the driving public and loved by traffic engineers. The multiple lane types require low speed and segregation of pedestrian crossings away from the vicinity. When the mix of traffic includes long wheel base trucks, the blind spots for clearance of merge maneuvers becomes an accident cause. The cure for the unseen car behind and to the outside of the truck is to increase the radius of the curve. This increase in area requirement is usually too expensive in Right-Of Way acquistion and brings justification for a grade separation interchange.

 

[TrafficDesigner]

Actually, what you are describing are the old traffic circles and not modern roundabouts, and I agree the old traffic circles did have some flaws to them and hopefully no one is still building those.

One of the most common myths of modern roundabouts is that they are the same thing as traffic circles and the general knee jerk reactions are as you have described. Modern roundabouts (when designed correctly) address the issues as you have mentioned.

What I was looking for specifically is standards or experiences with the truck aprons. I have seen a couple of general different ways to build them, but I was looking to make some comparisons and learn from what others have done.

 

[GeoPaveTraffic]

The one round-a-bout I have worked on included a truck apron. I no longer have access to the plans, but as I remember it the apron was about 4 feet wide and positioned 3 inches higher than the main part of the pavement. All drainage was 2 percent to the outside. At the transition between the pavement and apron a Missouri DOT Type N curb was used, basically a mountable curb with something like a 45 degree bevel.

The round-a-about is located in a residential area and is performing very well.

 

[LCruiser]

"New" roundabouts (weak excuses for traffic circles) are indeed a bane. We're even building one that's egg-shaped instead of round. They may have addressed some problems, but at the expense of efficiency. Having driven through them overseas literally hundreds of times, if you can't handle them just go another way. The 5% of drivers in the US who would not even be able to drive in traffic outside the country are the same whiners that create other inefficiencies (like "traffic calming").

 

[TrafficDesigner]

Again I would be willing to bet you are referring to the traffic circles. I am curious how an egg shaped roundabout meets the Modern Roundabout Standards. My guess is that you have an odd shaped intersection that the designer is calling a roundabout but it probably doesn't meet any of the standards of a modern roundabout.

Modern roundabouts when designed correctly control the speeds that a vehicle can enter and circulate through the intersection while at the same time keep traffic moving. When the standards aren't followed you end up with another traffic circle that has problems. Even the amount of traffic should be considered before the design even starts, otherwise it could fail from day one. I have seen intersections that should not have a roundabout or they are designed for a single lane when they should be a double lane roundabout or they are too close to a traffic signal. It all falls back on the designers and engineers to take the time to educate themselves on Modern Roundabouts and follow the standards or stop trying to design them.

When their design fails everyone gives the roundabout a bad name, when it technically is not even a roundabout but just another poorly designed traffic circle or circular intersection.

 

[LCruiser]

Do you have a reference to "Modern Roundabout Standards"?

Thanks...

 

[TrafficDesigner]

Yes, actually there is a lot of information out there now as opposed to several years ago.

http://www.alaskaroundabouts.com/links.html

Here is one link from alaskaroundabouts link page. They have a lot of good information.

Wisconsin DOT has a pretty good guide that was written by Scott Ritchie if I understood correctly.

http://www.dot.wisconsin.gov/safety/motorist/roaddesign/roundabout.htm

https://www.nysdot.gov/portal/page/portal/index

I learned a lot from Howard McKulloch of NYSDOT in one of his courses that he gave. He's the one that opened my eyes to the differences between modern roundabouts and traffic circles.

Arizona

http://www.dot.state.az.us/CCPartnerships/Roundabouts/faq.asp

Kansas

http://www.ksdot.org/burtrafficeng/Roundabouts/Roundabout_Guide/RoundaboutGuide.asp

Maryland

http://www.sha.state.md.us/safety/oots/roundabouts/safety.asp

These are a few and most of these have link pages that will take you to others.

 

[TrafficDesigner]

GeoPaveTraffic, That is the type of information that I was looking for. The most recent ones I have worked on the reviewers wanted to see a 20' paved area for the circulating traffic and then a 2' roll curb, with a 10' apron behind that and then vertical curb for the center island. The apron sloped 2 percent outward from the vertical curb towards the back of the roll curb.

It kind of looked like an upside down small plate or saucer.

 

[CygnusX1]

Wow, I am alarmed at the negative reaction to roudabouts, even among engineers. I suggest everyone make their peace with them: old fashioned, expensive, inefficient signalized intersections are being replaced with roundabouts every day. When my city installed its first roundabout, as expected, the public reaction was negative. Ten years later, most folks love them, and we have about a dozen finished and a dozen more in the works. I think it just takes a learning curve for the public to figure them out.

I'm designing a RA w/ 12' wide truck apron, determined from truck turns. The grading will be tricky because my site is not flat, so my circulatory roadway (and probably the apron and center island) will be a flat disc tilted at 4% grade.

It is red colored concrete, with a stamped brick pattern.

Curb on the apron is important...we use a 3 or 4" traversable curb (rounded to 1" radius) so the truckers are not shy in utilizing them. A bigger curb can cause them some trepidation getting their trailer up on the apron.

I am also an admirer of Howard McCulloch's methods. His NYSDOT website is very good.

 

[TrafficDesigner]

Thanks CygnusX1. Good information.

I was surprised at first by the negative reactions too, but then realized they weren't describing roundabouts, but rather the old traffic circles.

Your description of the truck apron is what I am hearing some of our local jurisdictions describing. I think they are leaning that way now.

I just attended another seminar this morning that Howard presented through ITE. I think engineers and designers should really consider attending one of his courses if they are going to design these. It certainly couldn't hurt.

Here is another link page that links to a lot of the D.O.T.s.

http://www.azroundabouts.com/Roundabout_Links.php

 

[tbone1]

Greetings,

We have been recently working on a roundabout that includes the "egg shaped" center island. Additionally the roundabout included a slip lane to account for a particularly high demand for right turns from one leg to another through the roundabout (separated by a raised median section for those not utilizing the immediate right turn (thereby eliminating a weaving pattern). The egg shape stems from the off tracking of trucks through the roundabout. Just some food for thought.

Tbone1

 

[TrafficDesigner]

Hey Tbone1, thanks for the input.

So does the egg shape design replace the truck apron?

I would love to see a .PDF of it if possible.

What is the circulating lane width, and did you have trouble with the smaller vehicles using it at faster speeds?

 

[Rob2004]

Yeah, a pdf of the design showing the laning information (schematic) would be great. Seems interesting.

 

[LCruiser]

Actually I prefer the old traffic circles to the modern roundabouts that I've seen. The roundabouts, while giving some advantage over signals in efficiency, are still inferior in traffic flow volume to the traffic circles I've used overseas. I view them as condoning incompetent drivers - much like "traffic calming". But hey, that's consistent with the rest of the dumbing down of America.

 

[ACtrafficengr]

I was involved in retrofitting a roundabout into the site of an old traffic circle. Peak hour delay went from over 1 minute per vehicle to less than 10 sec, crashes were cut in half, and injury crashes were cut by close to 80%.

I'm sure us in the US still have a lot to learn about designing roundabouts, but it's hard to argue with success.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[LCruiser]

"Traffic Circles" vary widely in design. If your "redesign" was in the US, it may have included stop signs to get in. The big difference in international traffic circles and "roundabouts" that I see are that traffic circles are multilane and can therefore handle more traffic with competent drivers. Unfortunately, in the US we tend to see driving more as a right than a privelege.

 

[TrafficDesigner]

ACtrafficengr, I agree it's hard to argue with the results that come from a retrofit or replacing a signal. I haven't done a retrofit yet but I would love to do a before and after study as you did, when I get the chance to work on one. My projects have been all new installs or modifying an existing one.

I would be interested any information you care to share on the location or design. One thing I have found helpful is to visit the intersection after 6 months or so and do an observation study. General things like tire marks on curbs, speeds on approach and circulating etc... I have learned that the center island treatment / apron curbing is pretty important in controlling smaller vehicle speeds while circulating.

 

[TrafficDesigner]

I have been seeing multi-lane roundabouts more and more now, usually in rural areas or along interstate exits, but when done right they can handle the traffic.

If you have an opportunity or if you haven't already done so, check out the new 2009 MUTCD. It has a roundabout section in it now. One of the differences between the multi-lane roundabouts and the traffic circles is that in the multi-lane roundabouts, you shouldn't be switching lanes in the roundabout. The driver has to make the decision to get into the correct lane on approach to the roundabout, and then stay in their correct lane as they pass through it.

I have been seeing more of this kind of striping on 2 laners.

 

[LCruiser]

I like the spiral striping - that looks clean. After looking at the 2009 MUTCD and the striping options, I've changed my mind, and like them. Ever used one that's oval though?