Background: Roundabouts were introduced to the Netherlands in the mid-1980s to provide a safe alternative to the traditional intersection. Today, there are approximately 3,500 fully functional roundabouts in the Netherlands. While the typical American traffic circle often evokes images of high-speed merges and deadly collisions, the Dutch roundabout can be characterized by low speeds, low stress driving, and a very low incidence of collisions. In the Netherlands it is estimated that the roundabout can reduce the number of severe casualties by 70% when replacing a four-way intersection. This high standard of safety is what makes the roundabout a cornerstone of Sustainable Safety.



Types: In the Netherlands, three important types of roundabouts are in use; the single lane roundabout, the un-signalized turbo roundabout, and the signalized turbo roundabout. Each roundabout is designed to accommodate different levels of traffic volume as well as the variable characteristics of the intersecting roadways.





Single Lane Roundabouts



Single lane roundabouts are the simplest and safest type of roundabout. These roundabouts have one driving lane for motor vehicles and can serve up to 25,000 cars per day. The single lane roundabout takes up the least amount of space of the different roundabout types, and can be found in the built up areas of communities all across the Netherlands. Single lane roundabouts can also be used in rural locations. Single lane roundabouts are also bicycle and pedestrian friendly, whereas turbo roundabouts are not.







Design Features



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Interrupt view of horizon. Principles: Predictability

As vehicles approach the single lane roundabout, the driver is alerted of the roundabout by an obstruction that abruptly cuts off his/her view of the horizon. The single lane roundabout shown in this photograph uses a large blue, red, and white sign to alert the driver of the roundabout.





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Near 90˚ entry. Principles: Homogeneity, Predictability

By making the entry to the roundabout close to a 90˚ angle, drivers are forced to slow down as they enter the roundabout, creating a safer overall environment within the roundabout. Homogeneity is established by slowing the entering vehicles down to the speed of the vehicles in the roundabout, and also to the speed of the cyclists. Predictability is inherent to this design as drivers are given a much clearer view of the vehicles inside the roundabout as they approach, and the forced slowing down at the near-90-degree entry reinforces the idea that entering traffic yields to traffic circulating on the roundabout.



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Apron for trucks.

By providing a curbed apron on the inside tract of the roundabout, trucks are able to mount the apron and make the wide turn necessary for them to navigate the roundabout successfully, and at a slow speed. Cars do not want to mount the apron and are thus confined to the narrow lane, keeping speeds low. The apron allows trucks to use the roundabout, but without widening the circulating lane so much that it would encourage the high speeds that would result if cars could find a nearly-straight path through the roundabout.



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Splitter island and zebra stripes for easy pedestrian crossing. Principles: Homogeneity, Predictability

Large zebra stripes make it obvious to drivers that they are approaching a pedestrian crossing, establishing predictability. Pedestrians are safe crossing the roundabout here because of the large zebra stripes and the splitter island (median refuge) that separates the two lanes of traffic.





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Bicycles and single lane roundabouts. Principles: Functionality, Homogeneity

There are no bike lanes in single lane roundabouts. On roundabouts where traffic is low, bicycles can simply enter the roundabout as the motor vehicles do. On single lane roundabouts where traffic exceeds 8,000 vehicles per day, a separate cycle track is build around the outside perimeter of the roundabout. Typically (and as shown in this image), cyclists are given priority throughout the roundabout.





Turbo Roundabouts



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Turbo roundabouts evolved out of a need for safer multilane roundabouts.The multi-lane roundabout was built to serve intersections where traffic volume surpasses the capacity of a single lane roundabout.These roundabouts are inherently dangerous, however, as confusion over which vehicle has priority when exiting the roundabout leads to frequent collisions.The design of the turbo roundabout allows for a high capacity of vehicles without compromising on safety.





Principles of the Turbo Roundabout



-As a single lane of traffic approaches the turbo roundabout, it is split into multiple turning lanes

-Drivers choose their turning lane (and ultimately their destination) before entering the turbo roundabout

-Once inside the turbo roundabout, there is no possibility for “weaving”

-Vehicles exit the turbo roundabout without any lateral conflicts

-Turbo roundabouts use spirals, not circles, to move traffic from entrance to exit





Types of Turbo Roundabouts



Turbo roundabouts can be classified as either a signalized turbo roundabout or an un-signalized turbo roundabout. Un-signalized turbo roundabouts can carry more capacity than a single lane roundabout, but not as much capacity as a signalized turbo roundabout. Un-signalized turbo roundabouts typically have 2-3 approaching lanes for a single lane of entering traffic, while signalized turbo roundabouts have 4-6 approaching lanes for each leg of entering traffic (though the departure leg never has more than has two lanes).

Principles: Functionality- established by providing different types of turbos for different traffic scenarios.





Un-signalized Turbo Roundabout



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Un-signalized turbo roundabouts are used when a single lane roundabout will not provide enough capacity for the intersection, but a signalized turbo roundabout is not needed. Notice how the single lanes of incoming traffic expand into multiple turning lanes as they approach the roundabout. Un-signalized turbo roundabouts typically use 2-3 entry lanes at each leg, and 1 or 2 departure lanes.





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Entering the un-signalized turbo roundabout. Principals: Predictability



As vehicles approach the roundabout they choose the appropriate lane to enter. As there are no traffic lights at this type of turbo roundabout, drivers must yield to vehicles already in the roundabout. Note the roundabout sign blocking the horizon and alerting the driver of the approaching roundabout, as seen in the single lane roundabout.

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Separate lanes within the roundabout.

Once inside the roundabout, drivers are discouraged to change lanes by raised dividers that separate the lanes. This prevents weaving and keeps drivers on track towards their destination. These raised dividers are what make 2-lane exits safe: the rightmost circulating lane is forced to exit, and the next inner lane gives drivers the option of exiting or continuing around the roundabout.







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Notice how far back the approaching lane begins from the roundabout. This gives drivers plenty of time to choose their lane, reducing congestion at the entrance of the roundabout.





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These two images show how a vehicle exits the un-signalized turbo roundabout. Notice how the vehicle does not have to cross any lanes of traffic, preventing lateral collisions.





Signalized Turbo Roundabouts



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Signalized turbo roundabouts have the highest vehicle capacity of any roundabout.These roundabouts can connect multiple, high volume, rural distributer roads.This roundabout connects 4 roads; one with 3 approaching lanes, two with 5 approaching lanes, and one with 6 approaching lanes.

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As vehicles approach the signalized turbo roundabout, signage guides them into their appropriate lane (as is done in the un-signalized turbo roundabout). The principal of predictability is clearly exhibited here, as drivers have plenty of time to read the signage and choose their lane. Here, there is one left approaching lane, two thru lanes, and two right approaching lanes.

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After vehicles enter their chosen approaching lane, they must pass through a traffic signal (not simply yield, as is done when entering an un-signalized turbo roundabout). The traffic signal has essentially a simple 2-phase operation (one phase for the North-South street, one for East-West), giving it a high traffic capacity per lane compared to a typical major intersection which has to split the cycle into 4 phases.



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Once inside the roundabout, vehicles stay in their respective lanes until they exit the roundabouts. Vehicles making a “left” turn start in the innermost part of the roundabout and spend the most time in the roundabout before exiting. Vehicles turning “right” start on the outermost part of the roundabout and spend the shortest amount of time inside, before exiting. Thru traffic travels between the “right” turning vehicles and the “left” turning vehicles. As in the un-signalized turbo roundabouts, vehicles travel in spiral geometry from entrance to exit.

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Cyclists pass unimpeded through a tunnel under the turbo roundabout. It is simply understood as part of the price of building a turbo-roundabout (which is a lot less than a grade separated interchange) that bikes and peds have to have an underpass.



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A vision for the future of bicycle-roundabout integration?

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