ITE Journal March 2018 - 47

For vehicles and cyclists, the magic behind roundabouts' life-saving
abilities lies in the self-enforced low speed entry and elimination
of right-angle interactions. For pedestrians, it is the low speeds,
separation between crosswalk and circulatory roadway, the crossing
refuge (which allows people to cross only one direction of traffic at
a time), and the direct interaction between driver and pedestrian.
The ability for a roundabout to save lives depends on these features
being present and well-designed.
Compared to a traffic signal, a roundabout is also usually more
efficient, longer lasting, and better looking. (Okay the last one is a
bit subjective.) A roundabout can cost more than a traffic signal, but
still competes well on value, and costs are dropping.

Key Message
As more roundabout designers learn how to reduce costs
without sacrificing performance, almost every roundabout will
be competitively priced. Did new roundabouts outnumber new
traffic signals in your jurisdiction last year? Will they next year?
Combined with their outsized value, a competitive price will set
roundabouts on a path to eventually replace most traffic signals in
the United States, even before those signals reach the end of their
lives. As quickly as this turnover occurs, severe injury and fatal
intersection collisions will become as rare in the United States as
they are in Sweden.2

San Diego, CA, USA Vision Zero Project Example

Other Cost-reduction Examples
Savings aren't limited to urban settings. This section provides a
wide range of examples of how designers can reduce roundabout
costs without sacrificing performance.
Pervious areas within the center island, splitter islands, and
sidewalk buffers can be used to handle storm water runoff, reducing
the cost of off-site mitigation measures if required.

King County

The City of San Diego, CA, USA is a Vision Zero city. The city's
first Vision Zero infrastructure project-selected by the California
Department of Transportation (Caltrans) and the Federal Highway
Administration to receive highway safety funds-is a set of three
roundabouts in a half-mile corridor that moves more than 20,000
cars every day. This corridor has one of the worst pedestrian safety
track records in the city, and the three roundabouts (replacing
two traffic signals in a coordinated system) will slow traffic

without hurting travel times and make it significantly safer for
pedestrians to cross.
The corridor is completely built out, so acquiring surrounding
property to accommodate the roundabouts is pricey and would have
delayed the project by at least a year-not exactly compatible with
the urgency of Vision Zero. The roundabouts therefore will all have
to fit within existing right of way. The intersections measure about
100 feet diagonally, so only a single lane will fit. They also have to
accommodate semi-trailers. Many hours were spent measuring
trucks on the corridor to learn what size trucks need to turn, and
which ones only go straight. The resulting layout will comfortably
accommodate all design vehicles, and the side street splitter islands
are fully mountable for unexpected semi-trailers (see Figure 1 of
a King County, Washington mini-roundabout that also uses fully
mountable splitter islands). Most of the vehicle traffic goes straight,
and the side streets are already low-speed, so side street deflection
is compromised in order to allow better speed control on the major
street than would otherwise be possible in these cramped quarters.
The three critical performance measures for this design are
major street speed control, major street capacity, and comfortable
accommodation of all design vehicles. The ability to compromise
less critical measures (like side street speed control and side street
capacity) allowed the design to meet the critical ones and fit in a
very tight space, costing far less and saving a year. Without this,
the project would not have included any roundabouts, hobbling its
ability to improve safety.

Figure 1. Example of a mini-roundabout at SW 100th St & 10th Ave SW, Seattle, WA, USA that uses fully mountable splitter islands.
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Table of Contents for the Digital Edition of ITE Journal March 2018

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http://www.nxtbook.com/ygsreprints/ITE/G110939_ITE_November2019
http://www.nxtbook.com/ygsreprints/ITE/G110110_ITE_October2019
http://www.nxtbook.com/ygsreprints/ITE/G110109_ITE_September2019
http://www.nxtbook.com/ygsreprints/ITE/G108559_ITE_August2019
http://www.nxtbook.com/ygsreprints/ITE/G108250_ITE_July2019
http://www.nxtbook.com/ygsreprints/ITE/G107225_ITE_June2019
http://www.nxtbook.com/ygsreprints/ITE/G104039_ITE_May2019
http://www.nxtbook.com/ygsreprints/ITE/G104038_ITE_April2019
http://www.nxtbook.com/ygsreprints/ITE/G104036_ITE_March2019
http://www.nxtbook.com/ygsreprints/ITE/G103582_ITE_February2019
http://www.nxtbook.com/ygsreprints/ITE/G102868_ITE_January2019
http://www.nxtbook.com/ygsreprints/ITE/G100155_ITE_December2018
http://www.nxtbook.com/ygsreprints/ITE/G100154_ITE_November2018
http://www.nxtbook.com/ygsreprints/ITE/G99495_ITE_October2018
http://www.nxtbook.com/ygsreprints/ITE/G98028_ITE_September2018
http://www.nxtbook.com/ygsreprints/ITE/G97366_ITE_August2018
http://www.nxtbook.com/ygsreprints/ITE/G96287_ITE_July2018
http://www.nxtbook.com/ygsreprints/ITE/G94315_ITE_June2018
http://www.nxtbook.com/ygsreprints/ITE/G93877_ITE_May2018
http://www.nxtbook.com/ygsreprints/ITE/G93065_ITE_Apr2018
http://www.nxtbook.com/ygsreprints/ITE/G91484_ITE_Mar2018
http://www.nxtbook.com/ygsreprints/ITE/G89434_ITE_Feb2018
http://www.nxtbook.com/ygsreprints/ITE/G86608_ITE_Jan2018
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