Launch Slideshow

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Advanced geometry

Advanced geometry

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    At one intersection the high groundwater elevation was estimated to be approximately 12 feet below the existing grade, so engineers were restricted to the depth they could lower the Keystone Parkway through lanes. Photos: American Structurepoint Inc.

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    By the end of the year, more than 60 roundabouts are expected to be opened to traffic throughout the 50 square miles of Carmel, Ind. Each will eventually have landscaping within medians and along the right of way.

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    Deploying a teardrop instead of a tight diamond design limited relocations to three homes and one business over a total of six at-grade intersection conversions.

ROAD: Keystone Parkway (SR 431) in Carmel, Ind.
LENGTH: 4½ miles
SIGNALIZED INTERSECTIONS: 7
BUDGET: $112 million
FUNDING: $90 million from state toll road leasing revenues, the rest from a bond issued by the City of Carmel
WEB SITE: www.carmellink.org

An ongoing project involving the nation's tightest-configured teardrop roundabouts has earned kudos, has reduced accidents with injuries by 80%, and is so effective that the Indiana DOT (INDOT) is considering the use of similar interchange configurations on some of its own projects.

An affluent community containing one of the state's largest and most successful business districts, Carmel, Ind., grew significantly over the last 10 years. The population of Carmel/Clay Township rose an average 2% to 2.5% annually; the city itself added 50,000 residents, 18,000 of whom are the result of annexing 16 square miles.

As usual, growth brought challenges as well as opportunities.

A heavily traveled road that bisects the city became increasingly sluggish and, as drivers tried to beat traffic lights, perilous. Since the late 1960s, Keystone Avenue had been a four-lane divided roadway with seven at-grade signalized intersections. According to an analysis performed by the state in the early 2000s, all but two classified as “failing” during peak travel periods. The corridor averaged 200 accidents/year and has had at least one fatality.

Typically, teardrop roundabout interchanges have a footprint similar to tight diamond interchanges, with at least 400 feet between ramp termini. But the interchanges on Keystone Avenue — now known as Keystone Parkway — have roundabouts with ramp termini of less than 290 feet. The city's systematic plan to convert several intersections into roundabouts most recently earned the 2009 Climate Protection Award from the U.S. Conference of Mayors.

A CONTEXT-SENSITIVE FIX

State engineers were planning to upgrade a parallel corridor — US 31 — in 2011 to interstate standards when they realized that Keystone would have to provide additional capacity while US 31 was under construction. They proposed adding a travel lane in each direction, adding turn lanes, and new traffic signals at intersections. Residents, however, strongly opposed the $50 million plan because it significantly increased the amount of paved area without resolving long-term congestion, safety, noise, and air quality concerns. It also didn't address what they felt was the community's greatest need: improved safety with east/west mobility for pedestrians and bicyclists.

City Engineer Michael McBride wanted a minimally disruptive, long-term solution.

Only two intersections pass through commercial areas; the rest of the corridor is heavily residential with backyards visible from and directly adjacent to the road. So McBride's team proposed an alternative — introduce grade separations at intersections using teardrop roundabouts — and Mayor Jim Brainard asked the state to relinquish control of the corridor, a request the city had been making for more than a decade.

This time the stars were properly aligned.

In 2006, Indiana Gov. Mitch Daniels made international headlines when he signed one of the nation's first and largest public-private infrastructure-leasing agreements. A joint venture made up of Spain's Cintra and Australia's Macquarie Infrastructure Group paid $3.8 billion for the right to operate the Indiana Toll Road for 75 years. The 157-mile road connects Chicago with Michigan and, according to the National Surface Transportation Policy and Revenue Study Commission, generated $88 million in toll revenues in 2005.

Indiana is investing the funds in an aggressive program, called “Major Moves,” of 400 road and bridge projects to be completed by 2015. One of those improvements, which is scheduled to begin construction this fall, is US 31.

The project development process for local agencies is much faster than the federal project development process that state agencies follow. Designing to local roadway standards also reduces project costs. Given the tight deadline, the state agreed Carmel was in a much better position to improve the corridor on time. In September 2007 the road was turned over to Carmel, which is now responsible for maintenance, snow removal, and right of way mowing.

American Structurepoint engineers broke in their VisSim software, developed by Visual Solutions Inc., to configure a design that would work within the project's narrow parameters.

The solution was to use unique tight teardrop interchange configurations, which are significantly smaller than conventional tight diamond or single-point interchanges. When measured center to center of the teardrop, the ramp termini are less than 290 feet apart. The maximum right of way width was held to less than 300 feet.

In addition, according to the Insurance Institute for Highway Safety (IIHS), roundabouts reduce personal injury accidents by 80% compared to standard signalized and stop-controlled intersections.

“Anytime you remove conflict points, you improve safety. A two-lane, four-way, signalized intersection has more than 30 conflict points, but a typical single-lane roundabout has only eight.” McBride says. “Having Keystone run below cross-street roundabout intersections, used for ingress and egress to Keystone, will improve safety exponentially.”

The new configuration improves corridor progression and decreases travel times from one end of Keystone to the other now that the signals along Keystone have been removed. When the final interchange is completed, motorists will be able to travel between 98th Street and US 31 without being impeded by a signal. Travel times on the side streets are also reduced since motorists do not have to stop for a signal; the roundabouts on the side streets require a vehicle to slow to 15 mph, but drivers will not have to sit through excessively long traffic signal cycles. Instead, they just have to yield to vehicles within the roundabout.

Using the program to model driving behaviors and see the impact of various designs on traffic flow, engineers reconfigured Keystone Parkway to provide a service level of A or B (the top two levels of service on a scale of A to F) without having to add through lanes.

The parkway was lowered as much as 25 feet while still maintaining four lanes of through traffic. For the 106th Street interchange, engineers were able to lower the road only about 8 feet. But at Main Street, crews are excavating as much as 25 feet.

An alternative bid scenario that allows the construction contractor to close the road for up to 45 days is saving considerable amounts of money.

MONEY-SAVING CONTRACT OPTION

The winning bid on 136th Street interchange for the Keystone closure alternate was $700,000 less than the base bid, while the closure alternate bid for Main Street came in around $500,000 less. The Carmel Engineering Department expects to save more than $1 million — and minimize disruption to a high school with 4,100 students — on the two interchanges.

The first two interchanges were designed in approximately four months, and construction took a year. The third interchange was let in fall 2008 and finished last August. The final project is expected to be completed and open to traffic this fall.

One of the greatest benefits of eliminating signals is the added east/west connectivity that will be provided for motorized and nonmotorized transportation. Keystone will no longer act as a barrier between east and central Carmel.

“The design makes it more pedestrian friendly and drivers will no longer wait in long lines for stoplights to turn green. This context-sensitive solution has provided a very aesthetically pleasing result for the surrounding neighborhoods,” Mayor Brainard adds.

— Craig Parks (cparks@structurepoint.com) is a project development director with American Structurepoint Inc.