Minneapolis ITE: roundabouts and ramp meters

Ramp metering and roundabouts were major topics at the recent annual conference of the Inst of Transp Engineers (ITE) in Minneapolis. On a crowded expressway there are major benefits to ramp metering — average speed and capacity in the mainline increases 25%. That's because vehicles on the ramp are 'metered' by a signal to enter the expressway one at a time and manage to find gaps in the mainline traffic without disrupting it. The worst thing you can do for an expressway is to send a whole platoon of vehicles up the ramp. It is almost like closing a lane!

Yet that is exactly what traffic engineers do when they install traffic signals at the interchange ramps in the conventional signalized 'diamond' interchange. The intersection signals store vehicles approaching the expressway, form them into disruptive platoons and send them up the ramp together to create maximum merging havoc in the mainline! That is the havoc that according to Glen Carlson of Minneapolis' Traffic Management Center creates shockwaves on the mainline that lead to inefficient and accident generating stop-and-go traffic. The Twin Cities with perhaps the world's most intense use of ramp meters have noticably smooth and freeflowing traffic on their expressways. Carlson says that their expressway grid regularly carries 2400 vehs/lane/hour whereas in expressways suffering the assaults of platoons on the ramps the mainline lanes begin to suffer 'shock' conditions as low as 1800 vehs/lane/hour.

The only trouble in Minneapolis is that it becomes a major exercise to 'board' an expressway because of the combination of signalized intersections and ramp meters. Unless you are lucky with a green at the approach intersection (odds are about 25%) you have two signal waits, first at the signalized intersection, second at the ramp meter. Roundabouts act as both intersection controllers and ramp meters in one wireless package.

The hot topic in one overflow session at ITE this year was roundabout interchanges for expressways. According to ITE papers roundabouts are a win-win substitute for signals at the intersections of expressway interchanges — providing safer and smoother traffic flows for the expressway at low cost.

Let's get back to basics: a signal brings out the gambler in drivers. The green signal phase is almost hard-wired to the accelerator of approaching vehicles, commanding them progressively to speed up lest they be punished with a red, and a long wait. And before the red of course there's an amber or yellow, and that 'Do-I-floor-it?' or 'Do-I-brake?' dilemma that engages the driver who also has to worry about being rear-ended if he is over-cautious. By contrast a roundabout with its splitter islands and raised circle is a reassuringly solid and unambiguous constant in the driver's mind. You see it stuck there solid, an obstacle to be negotiated in the middle of the intersection and there's no doubt about what you must do. You have to slow. By forcing each entrant to the intersection to deflect right (left in UK, Japan, Oz) in order to avoid hitting its curb/kerb a well designed traffic circle slows drivers to a safe speed. And conflict points with other motorists are reduced and simplified to finding a gap in circulating traffic to enter.

The roundabout is an American invention, attributed to traffic pioneer William Phelps Eno early in the century, and he introduced them to Europe. Winston Churchill as a young parliamentarian cursed the early roundabouts in London as a crazy American idea which he declared in ringing nationalistic terms had no place in Britain, having, some said, recently been pulled over by a cop for going the wrong way round a new Eno-inspired circle. With prevailing offside priority (Give-way-to-the-right rules in U.S., and Europe) the early circles locked up in heavy traffic since they required circulating traffic to give way to entering traffic. Gridlocked circles became a prop of comic movies in which busloads of baton-wielding, whistle-blowing police were dispatched to quiet and untangle stranded masses of angry fist-shaking, tooting motorists blocked in circles. It took until the 1960s to institute the simple fix in Britain — priority for traffic inside the circle over entering traffic by the simple expedient of placing 'Yield' or 'Give Way' signs on entries. Europe followed. At the same time the modern roundabout, a quantum leap improvement over old cricles was evolved — a mix of scientific studies. trial & error, and commonsense, like most good engineering.

Spectacular improvements in safety have been achieved at many thousands of modern roundabouts in Britain, Netherlands, Australia, and in the U.S. with injury accidents reduced 70 to 80% below average levels that prevail on open intersections with priority rules and half the level of crashes at signals.

Because roundabouts flow continuously with most traffic only having to slow to enter, and any stops short, they are far more efficient in moving vehicles per square yard of pavement in most circumstances than signalized intersections. Modelling of a new interchange on the Washington Beltway (200,000 veh/day) at Ritchie Marlboro Road in Prince George's Co MD by Hurst-Rosche Engineers showed one with roundabouts at the ramp ends would provide about 66% greater capacity with spectacularly smaller delays per motorist (one-tenth the delay) compared to a conventional signalized diamond. At Vail Road and I-70 Colorado, the main entryway to the famous ski resort, motorists coming in from Denver used to back up hundreds of yards onto the mainline lanes because of the delays at the conventional stop-signed (peak hours police-directed) diamond interchange below. There were waits of up to 20 minutes and Vail-bound motorists knew to take remote exits congesting local streets. The standard U.S. improvement at Vail Rd/I-70 was multiple ramp lanes for storage and a complete rebuild of the twin bridges of the mainline in order to accomodate 4 or 5 lanes underneath needed for a high capacity signalized diamond — a $10m-plus job. A singlepointer would have been even more costly. Roundabout engineers Ourston & Doctors of Santa Barbara CA designed a twin roundabout solution for the ramp ends that works with the existing 2-lane underpass and only cost $2.5m (see plan.) It has been so successful and popular that two more roundabout interchanges are in the works for I-70 in the Vail area.

Hal Kassoff who is just leaving the top job at the Maryland State Highway Administration for a senior position at ITS America has pushed roundabouts strongly in this state. Maryland's major new expressway going into construction next year — an 8km westward extension of MD-100 in Howard County — has three roundabout interchanges (MD103, MD104, Snowdon R. Pkwy).

Turnpikes under pressure to improve interchange performance to levels of 4,000 or 5,000 veh/hour off the mainline should include roundabout designs in their alternative studies. By reducing the need for turn lane and ramp storage, and hence expensive bridging, they can often reduce construction costs drastically. The lack of signals reduces maintenance costs, and roundabouts naturally meter ramp traffic headed onto the mainline. (Contacts: Ourston & Doctors CA 805 683 1383, Hurst-Rosche PA 717 428 3330, Andrew O'Brien & Assoc. Australia 613 9882 9955)