LOW TECH: Roundabouts endorsed by FHWA

Roundabouts (rdbts) have been endorsed by a senior FHWA research engineer as a more cost-effective treatment of intersections than signals or stop signs. Many toll road authorities could usefully add to their record for safety and increase capacity by replacing diamond interchanges (IC) with rdbt pairs or “dogbones,” one rdbt for each ramp pair. Partial clover-leaves would benefit from similar treatment. Whenever signals are being contemplated to replace Stop signs because of high volumes on ramps and the mwy crossing road, expensive widening of the bridge or underpass has to be contemplated to provide left turn storage lanes extending over or under the mainline. In these situations the double rdbt ‘dogbone’ design can be a huge money saver. Its free flow nature obviates the need for long turn-storage lanes, and the associated wide bridge and underpass structures. Joe Bared of the Turner-Fairbank Research Center in the cover story of the latest TR NEWS (#191 July/Aug 97 p13) from the Transp Research Board lists both signalized and unsignalized intersections at motorway interchanges as situations where major safety and capacity increases can be gotten with rdbts.

FHWA has been systematically reviewing experience at rdbts and is sponsoring a number of efforts to develop new modelling, and to document state of the practice. Bared’s article shows the very significant reduction of accidents that could be achieved with rdbts — approx 30% to 60%. Roundabouts work best in situations such as diamond interchanges where there is a high percentage of turning traffic. They need a rather flat site, and of course circle space. They work least well where side flows are extremely light and they can cause problems for cyclists. The modern rdbt is a highly developed design, quite different in important detail from old traffic circles. In Britain they are standard motorway IC treatment though most numerous there are the single large circle type which spans the motorway on two bridges, whereas the safest and most economical design is the “dogbone” of two smaller circles on either side of the mainline and a single slim bridge or underpass between. Rdbts are being deployed at a rapid rate in France and Holland and there is considerable interest in them in Germany and other European countries and in Asia. They are being built in small numbers in in CA, FL, MD, ME, VT, FL, UT, CO, MI, KS in the US and in BC in Canada.

Colorado's Vail Valley is the US showcase of modern roundabouts, with 9 high-capacity rdbts at former traffic congestion hot spots in a 15km corridor. 3 dogbone rdbts eliminated congestion at 3 ICs in Vail and nearby Avon, where in the ski season the inadequacies of diamonds caused traffic to back way up the ramps occasionally spilling back dangerously onto the mainline of I-70. Three other rdbts have eliminated congestion in downtown Avon, replacing traffic signals. I-25 at SR-52 north of Denver also may be getting a dogbone rdbt IC.

Ozabout vs Eurobout: There’s an interesting doctrinal schism in the world of rdbt design between Australian and European camps, each of which have their adherents in the US. Australian doctrine says rdbts work at up to about 4,000 vehs/hr capacity after which you put in signals, whereas the European doctrine is that they work well way beyond that. So far the Australian-rdbt camp is ahead in the US with FHWA’s Joe Bared and Florida DOT saying in an ‘strine accent “No good over 4,000, Might!” However Ourston & Doctors of Santa Barbara CA, the leading US rdbt design house (they’ve built 13, have 7 on the go and 20 more in preliminaries), is an exponent of the European rdbt doctrine which argues for a broad application at virtually all kinds of intersections.

Leif Ourston says one of the Vail CO rdbt’s has a capacity of 5600/hr and with huge volumes of visitors successfully using the dogbone interchanges for the first time in Rocky Mountains winters they have proven how quickly they are adopted by American motorists. Higher capacities are possible, he says — not by widening the laneage in the circle as you might imagine but by splaying the entries to increase storage.

Actually Joe Bared FHWA says the Oz/Euro differences over rdbt capacity are diminishing with new modelling and the rapid spread of different rdbt configurations. With about 14k Americans killed in intersection smashes each year, half at signals, and a proven safety improvement of 30 to 60%, the rdbt potential for saving lives is 4k to 8k. State DOTs and city engineers are starting to take modern rdbts seriously and toll road designers should too. (Contacts Joe Bared, Turner Fairbank 703 285 2509, Leif Ourston, Ourston & Doctors 805 683 1383 www.west.net/~owendee)