Case Study: Roundabout Interchanges

Roundabout Valley

The Colorado Rocky Mountain towns of Avon and Vail did what many of us wish our own home towns and cities would do. They eliminated traffic congestion.

They built high-capacity modern roundabouts at nine intersections which had caused long delays. Six roundabouts replaced Interstate 70 ramp intersections formerly regulated by STOP signs, and three roundabouts replaced signalized cross intersections in downtown Avon.

Now there are three modern roundabout interchanges in the Vail Valley, about one hundred miles west of Denver, Colorado. With these major roundabouts, the Vail Valley has become America's “Roundabout Valley,” the showcase for high-capacity roundabouts on this continent.

North America Now: Hurry Up and Wait

Traffic on North America's arterial streets may be characterized by four words: hurry-up-and-wait. Motorists accelerate from one signal and hurry to the next one, only to stop for two minutes at a red light. The average speed along a 45 mile-per-hour road may be as low as 15 miles per hour when stopped time at red lights is considered.

A proliferation of STOP signs is another major factor in North America's hurry-up-and-wait system. STOP signs are often installed where, because of ample sight distance, prudent drivers do not stop; we simply yield. We slow to see whether vehicles are approaching on the through street, but we do not completely stop unless there is a reason to stop. YIELD signs would improve compliance with the law at these high-visibility locations. Roundabout Valley eliminated 37 STOP signs. This is suggestive of North America's future.

Britain Now: Always Flowing

By contrast, British traffic may be characterized by two words: always flowing. Although at peak hours Britain has long delays at some over-capacity intersections, during off-peak hours one can drive British surface roads through cities, suburbs, and towns for many miles without stopping. British intersections are predominantly T-intersections, each regulated by one side-street YIELD sign. Major intersections are roundabouts, regulated by the yield-at-entry rule. These two types of low-delay intersection keep traffic flowing. STOP signs are used very sparingly, at intersections having impaired sight distance.

The four-way cross intersection, the favorite type of intersection in North America, has been rendered obsolete in Britain. It is superseded by the roundabout, which carries heavy flows of traffic with greater safety and efficiency. Avon, Colorado, has replaced every cross intersection on Avon Road with a roundabout. This is suggestive of North America's future.

Study Roundabout Valley

In a nine-mile area the highway researcher will find roundabout variety not available anywhere else in the continent:

  • 9 high-volume roundabouts.
  • 3 roundabouts that have replaced signalized cross intersections.
  • 4 raindrop-type roundabouts.
  • 4 full-circle roundabouts.
  • 1 oval roundabout.
  • 8 right-turn bypass lanes.
  • 25 flared entries.
  • 4 one-lane entries.
  • 22 two-lane entries.
  • 8 three-lane entries.
  • 6 freeway off-ramp entries.

Roundabout Valley chart

Lessons that can be learned from Roundabout Valley:

  • The modern way to eliminate traffic congestion is to build roundabouts at the most heavily impacted intersections.
  • Roundabouts eliminate congestion, leaving beauty in its place.
  • Roundabouts are safer and more efficient than traffic signals.
  • Roundabouts are safer and more efficient than cross intersections.
  • It usually costs much less to widen nodes than to widen links, especially where links are expensive, as at interchanges, at the ends of tunnels and bridges, and through built-up areas.

I-70/Vail Road, Colorado, 1995

The Town of Vail, Colorado, built the first roundabout interchange in North America at Interstate 70/Vail Road in 1995. Closely spaced ramp and frontage road intersections, formerly regulated by STOP signs, were replaced by two roundabouts, with two- and three-lane entries. The six-leg south roundabout has an inscribed circle diameter (ICD) of 200 feet and a capacity of 5,200 vehicles per hour; the five-leg raindrop-type north roundabout has an ICD of 120 feet and a capacity of 2,700 vehicles per hour. The work was guided by the application ARCADY / RODEL.

Photo of Vail Road

The total cost of the project was $2.8 million. The project saves the town $85,000 per year on traffic direction officers. Long queues, which used to extend back onto the freeway, now rarely exceed ten vehicles.

The crash rate for the first year of operation dropped to 22 crashes in the after period from a yearly average of 25 crashes in the before period. Injury crashes decreased to three from an average of five. The project received a high approval rating, 4.4 on a scale of 5.

I-70/Chamonix Road, Colorado, 1997

Following a series of twelve neighborhood meetings in 1996 to consider options for Vail's second most impacted interchange, I-70/Chamonix Road, it became clear that residents wanted a modern roundabout interchange like the one built a year earlier at I-70/Vail Road. The new interchange was designed and built in one year, light speed for highway engineering projects. Opened in the summer of 1997, the roundabouts have eliminated traffic congestion at the interchange.

Chamonix Road

Each of the two roundabouts has an inscribed circle diameter of 150 feet, and each has six legs, two bypass lanes, and two-lane entries. The peripheral pedestrian-bicycle road crosses Gore Creek twice. The north roundabout has a capacity of 3,700 vehicles per hour, and the south one has a capacity of 3,300 vehicles per hour.

Avon Road, Colorado, 1997

In Avon, Colorado, a town nine miles southwest of Vail, voters increased their own property taxes to pay for North America's longest wide-node-narrow-link road. Five roundabouts relieve traffic congestion along the entire length of Avon Road, the only connection to Interstate 70 for the towns of Avon and Beaver Creek. The roundabouts were opened in the fall of 1997.

Avon Road

I-70/Avon Road has two 150-foot-diameter raindrop-style roundabouts. Entries to the north roundabout have three-lanes, and entries to the south roundabout have two-lanes. Three right-turn bypass lanes boost the capacity of the interchange to 4,200 vehicles per hour through the north roundabout and 5,800 vehicles per hour through the south roundabout.

The southerly three roundabouts have replaced three signalized cross intersections:

  • At Beaver Creek Road, an oval roundabout with 75-foot-radius tips and 200-foot-radius sides has a capacity of 6,000 vehicles per hour. Entries have two and three lanes.
  • At Benchmark Road, a 150-foot-diameter raindrop-style roundabout built on a sharp crest vertical curve prevents circulation around its low side. If left open to full circulation, a five-percent adverse cross slope on the low side would be hazardous to trucks. Drivers of fire and police vehicles can remotely actuate a lift-gate through the tip of the central island. With two-lane entries, the roundabout has a capacity of 4,300 vehicles per hour.
  • At U.S. Highway 6, a 150-foot-diameter roundabout with two-lane entries has a capacity of 4,900 vehicles per hour.
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