Mike on Traffic

Roundabout Rule of Thumb for Preliminary Layout

Roundabouts are becoming increasingly popular.  Given their operating characteristics, I think they're better than traffic signals for controlling traffic at the main entrance into most developments.  Signals work best at intersections that have high traffic volumes over 8+ hours a day.  A lot of entrances into major developments (whether they're residential, commerical, or office) have big peaks in traffic for a few hours of the day, but often don't have sustained traffic throughout the day.  At off peak times roundabouts cause a lot less delay than signals.

A lot of the civil designers and landscape architects who put together the site layouts for major developments don't have experience with roundabouts.  Below is a sketch that can help site designers put together a rough layout for the entrance intersection (info is based on a class I took taught by Ourston Roundabout Engineering).  There's a lot of tweaking that will happen in the final design of the roundabout, but just knowing the rough diameter of the circle is enough to move ahead with the internal design of the site.

Roundabout Technical Workshop - OregonITE

  Here is a guest post from Micah Heckman of Lancaster Engineering in Portland, OR (Thanks Micah).  It's a quick report on a workshop he recently attended on roundabouts.

The roundabout workshop was somewhat useful although it may have been more useful for roadway designers.  They did have some discussion on the 2010 HCM but they did not have any examples as I was hoping.  A few things I did come away with: 

-          NCHRP 3-65/Report 572 is the basis for HCM 2010 but HCM 2010 will move beyond the NCHRP report.

-          The LOS output

o    will be based on control delay

o    will be assigned to each lane, approach, and intersection

o    delay thresholds will be the same as for TWSC and AWSC intersections

One interesting point of note was from a presentation by a representative of the City of Bend, Oregon which is a large proponent of roundabouts (I believe 28 installed since 1999 in a town of 82,000 people).  They, along with Kittelson & Assoc. worked on locally calibrating the roundabout model and found that in Bend, the critical gap was 1 second less that reported in NCHRP 572 (4.1 sec vs. 5.1 sec) and the follow-up headway was 0.5 seconds less (2.7 sec vs. 3.2 sec).  It appears as if this is a result of experience using roundabouts.  

It will be nice once HCM 2010 is released to be able to have this analysis alternative readily available.  We currently don’t do a whole lot of work with roundabouts but there are certain instances (a new development with no right-of-way restrictions) that would a good location for a roundabout.

 

Negative Economic Impacts of Roundabouts?

Here is an article from Binghamton, NY briefly discussing the political posturing over putting a roundabout in their downtown.  Apparently the local business owners believe the roundabout may kill their businesses.

Reconstructing streets in downtown areas is often an economic negative for the business owners.  When I was an intern for the City of St. Paul about fifteen years ago, a coffee shop owner got a fair amount of press for coming out and throwing donuts at the equipment reconstructing the street in front of the restaurant.  The coffee shop ended up closing.

Setting aside the economic downturn suffered during construction, I am wondering if anyone has studied the economic impacts on businesses of roundabouts versus traffic signals.  I haven't come across such a study. 

I occasionally cite research done by Iowa State that points to the economic impacts of limiting driveways.  It is helpful to have research to fall back on instead of arguing about opinions.  I would like to have research to cite on the roundabout/traffic signal economic debate.  Any academics out there that can take this up?

Roundabout Primer Courtesy of Time Magazine

Jane and I were out to dinner with friends Saturday night and one of them asked me (this is a professional hazard for traffic engineers) "what's the deal with all of the new roundabouts?"  Check out  Time's one page article on roundabouts for a succinct answer.

source:  Time Magazine

Roundabout Experience in Minnesota

Micah commented on one of my blogs asking how Minnesotan's are reacting to roundabouts.  At the time I said it wasn't much of a news story.  I stand corrected.  Here is a link from an article in today's Star and Tribune.  Highlights:

• Safety benefits of roundabouts are tracking national trends - mostly fender benders, not injury crashes.
• Motorists are somewhat confused about driving in roundabouts, especially two lane roundabouts.

This is great publicity, but we need more.  I wish the Department of Public Safety would undertake an education campaign on how to drive a roundabout. 

Tangent - I haven't been able to track down if roundabout driving is being taught in driver's ed classes.  Does anyone know if it is? 

Safety Measures for Roundabouts

The Insurance Institute for Highway Safety came out with a report evaluating safety at existing roundabouts in Maryland.  They found that most crashes, whether at single lane or double lane roundabouts, occurred on the approaches into the roundabouts not within the circulatory part of the roundabout.  In a nutshell, they recommend better landscaping within the middle of the roundabout along with better signing/pavement markings in advance of the roundabout.  Both measures are to clue drivers in that there is a roundabout ahead before it is too late for them to slow down.

The report did confirm the safety benefits of roundabouts.  Only 25% of the crashes in Maryland at roundabouts between 1993 and 2005 involved injuries.  More importantly, there were no fatalities at the 38 roundabouts studied.  This isn't a large sample, but the safety track record is encouraging compared to the safety at intersections controlled with traffic signals.  According to the Minnesota Office of Traffic Safety there were 25 fatal crashes at traffic signals in Minnesota in 2006.  A very low number, but maybe a few of them would have been avoided with roundabouts.

Mr. Traffic Engineer - What is Level of Service?

Traffic engineers grade the operation of an intersection as A, B, C, D, E, or F based on the amount of time each vehicle has to wait to go through the intersection during a particular hour.  We use grades to give non-engineers a feel for how the intersection operates.  We don't want to get hung up on fractions of seconds.  Every jurisdiction I know of considers Level of Service (aka LOS) A, B, and C to be acceptable.  Most jurisdictions in Minnesota considers LOS D to be acceptable, but LOS E and F are both unacceptable.  Some places, like Los Angeles, consider LOS F ok depending on how bad it is (the seconds of delay calculated).  You need to check with the jurisdictions who control the intersection.  I have been burned a few times by "unique" standards different jurisdictions have in their policies or ordinances.  Checking the local standards is now one of the first things I do on a new project.

For you technical types, the chart below shows the amount of Control Delay (in seconds) that makes up each category.  These are defined in the Transportation Research Board's 2000 Highway Capacity Manual.

                                       

Level of   Service	Intersection   with     Traffic Signal	Intersection   with     Stop Signs
A	≤10   sec	≤10   sec
B	10-20   sec	10-15   sec
C	20-35   sec	15-25   sec
D	35-55   sec	25-35   sec
E	55-80   sec	35-50   sec
F	>80   sec	≥50   sec

Note - the chart is different for traffic signals and stop signs.  This reflects people having more patience sitting at a traffic signal than a stop sign.  To date, there are no Level of Service standards for roundabouts.  Most traffic engineers fudge this by applying the delay table for stop signs to the amount of delay calculated by software programs such as VISSIM, Rodel, or Arcady.  It is not an apples to apples comparison.  Hopefully the next edition of the Highway Capacity Manual will address Level of Service at roundabouts.