INTERCHANGES:Roundabouts endorsed by TRB panel

Roundabouts (rdbts) are one of those intersection control devices where the devil is in the details. Well-done they beat signalized intersections hands-down in both safety and vehicle throughput, and can be an esthetic and environmental enhancement to boot. Poorly done they are a fiasco.

The Transp Research Board’s (TRB) “Modern Roundabout Practice in the United States” is a bit of a misnomer because there isn’t that much modern roundabout practice in the US to write about. The report, written by Georges Jacquemart, a New York City engineer, sensibly devotes a good portion of its space to a review of roundabout design in Europe and Australia where there is far more contemporary experience. The report says the US has about 50 modern rdbts vs 35k elsewhere.

Gyratories or circles as an intersection control device are an American invention that was taken to Europe 80 years ago by the New York traffic control pioneer William Phelps Eno. Winston Churchill while a young parliamentarian denounced them (he’d been ticketed by the cops for taking a London one the wrong direction). He told the British parliament they were a new-fangled American idea which would never be accepted by the British. In the 1930s and 40s they sprouted all over the northeast US. Some of the old designs linger on in Washington DC and New Jersey, but they are an embarrassment to advocates of modern circles. The old ones were hampered by yield-right rules that favored entering traffic over vehicles already in the circle so they got locked up — real, literal gridlock that had to be sorted out by sending a busload of cops to “unlock” the circle. And when they flowed freely, their large size encouraged excessive speed so they weren’t very safe.

Much safer than signals

The new ones are smaller in diameter and have entries designed with a carefully designed deflection to the right to force traffic to slow to c20mph. The righthand curb is angled to force entering traffic to looks for gaps in the in-circle traffic rather than doing a merge and weave as on the old larger ones. The TRB report surveyed 11 US rdbts and found they have reduced injury accidents about 50% as compared to signals or ‘Stop’ signs. Also, with only momentary stops and usually rollthroughs, throughput is so much better than at signals that delays are an average of less than quarter — a major reason there is such a saving in storage lane pavement.

“Slower speeds (forced by the rdbt) make drivers more aware of their environment and of other users. Yielding to traffic in the circle induces a higher degree of responsibility (in) the driver as compared to the ‘go’ message (of) a green light... To conclude, rdbts can have significant benefits in safety, (reduced) delays and (improved) capacity. Another major new benefit is the esthetic and urban design improvements resulting from the landscaping and sculptural elements in the central island. Rdbts can bring a sense of place to an intersection...”

While there is often skepticism among the public about modern rdbts beforehand, everywhere they have been installed in the US, drivers like them. In Montpelier VT, for example, public opinion was 68/14 against a rdbt in the main street before it was installed, but 73/0 in favor (balance neutral) after it was in operation. So, installing a rdbt requires forceful gutsy leadership beforehand, but is likely to bring accolades afterwards.

Most of the rdbts installed so far in the US are on local and arterial surface roads. The only rdbts in use in conjunction with a motorway are at the large ski resort of Vail CO on I-70 where there is a “dogbone” design with a pair of rdbts, one circle on each side of the mainline where the ramps converge with Vail Rd, the cross street.

Maryland will soon have several rdbt-pairs in conjunction with motorways. MD-100 due to open before the end of 98 has 3 rdbt interchanges. I visited them the other day. They’re complete with cobblestone-like truck aprons and landscaped with prolific day-lillies and cherry trees in their central circle. MD’s US-29 which is being upgraded to mwy standard will have another two dogbone pairs in southern Howard Co at MD-216 and at Johns Hopkins Rd (a neat low tech solution alongside the ITS guys at the Applied Physics Lab nearby.) The city of Frederick, here, is to have rdbts on either side of I-70 in the rebuilding of interchanges along its southern border.

Beltway to get dogbone IC

But the highest profile will be a dogbone pair of rdbts at a new interchange on the Washington Beltway (at about its most easterly point) at Marlboro-Ritchie Road in Prince George’s Co MD (see above.) The state’s rdbt expert Mike Niederhauser of MDOT says a dogbone pair of rdbts was the clear winner in a set of alternate designs, which pitted the rdbt pair against a clover-leaf and a signalized diamond. Estimated costs were: clover-leaf $65m, signalized diamond $50m and the pair of rdbts $25m. Much of the cost savings over the signalized diamond is because the rdbts allow the existing Beltway over-bridges to be left intact since they work with 4-lanes underneath. With the signalized diamond 6 to 8-lanes are needed under the Beltway to provide turning storage lanes, requiring a complete rebuild of the Beltway bridges. But Niederhauser says the rdbts perform better too. The clover-leaf has weaving problems of entering and exiting traffic while the diamond with signals, even with double turning lanes gets congested more quickly because of the long signal cycles, and is likely to have a higher accident rate.

At MD’s five rdbts operating 4 years or more, there has been an 86% drop in injury accidents, a 63% drop in total accidents, and not a single fatality. There has been very strong public acceptance, Niederhauser says, and he has lost count of the number that are now under study or in design in the state.

Ed Meyers of Hurst-Rosche Engineers who is doing final design work on the Marlboro-Ritchie Beltway rdbts says they will each be 67m (220’) outside diameter, providing two in-circle lanes and a 3.6m inside truck apron which can be removed later to increase capacity to three in-circle lanes.

Guidelines

There are official design guidelines for rdbts in MD and FL, and the specialist rdbt consultants Ourston & Doctors sell their own “Guildelines” and there are British and Australian manuals and software. The FHWA has started on a Rdbt Guidelines document due out in the Fall of 99. (“Modern Rdbt Practice in the US: A Synthesis of Highway Practice,” NCHRP Synthesis 264, National Academy Press, www.nas.edu/trb. Contacts: Georges Jacquemart 212 353 7474, Ed Meyers 717 428 3330, Ourston & Doctors www.roundabouts.com)

MESSAGE: Paul ‘Signal’ Box and kindred anti-roundabout crusaders, take note. The enemy is congregating October 1 & 2 in Loveland CO. A single well placed bomb could wipe out most of the world’s experts in the infernal rotaries: Barry Crown, John Peirce, Lief Ourston, Rahmi Akcelik, Michael Wallwork, Ed Meyers, Ed Waddelle, Peter Doctors, Georges Jacquemart... the whole darned lot of ‘em will be there preaching their misguided cirulatory theories to the innocents. It’s the ITE-Colorado branch’s “International Conference on Modern Roundabouts.” To find out more about where these plotters of dizzy traffic tricks are congregating, for the sake of signals, call Bill Cottrill 303 969 2175 fax 303 987 6995 bill_cottrill@nps.gov