4.1b Single lane roundabouts

Single Lane Roundabouts

Systematic Safety

Systematic safety is an approach to planning and designing transportation facilities based on the belief that mobility is a basic human right.  Following this reasoning, people of all ages and capacities should have at least one mode of transportation made accessible to them, with their safety being ensured at all times.  The systematic safety approach has developed in the Netherlands alongside the Vision Zero program, which aims to eliminate all fatalities due to traffic collisions among pedestrians, cyclists, and drivers. Rather than improving traffic safety as a reactive measure at black spots with particularly high fatality rates (much as we still do in the US), the Netherlands has systematically improved their transportation network to eliminate the opportunity for accidents while giving special consideration to the inherent vulnerability of pedestrians and cyclists.  Systematic safety recognizes that all human beings are prone to making mistakes, and therefore both preventative measures for dangerous behavior and forgivingness for when errors are made are strongly considered in transportation design.

Single Lane Roundabouts

A single lane roundabout is both the most efficient and the safest design to handle a steady flow of traffic at a 3 or 4-way intersection with one lane entering per direction. Rather than relying on signal technology to instruct drivers where and when they may drive, a roundabout simplifies the design of an intersection so that it is almost impossible to make a mistake even without signals. All cars entering the roundabout do so by making a right hand turn, and all cars exiting the roundabout do so by making another right hand turn, forcing traffic to flow in a single direction.  Effective signage makes it clear to all cars entering the roundabout that they must yield priority to cars inside the roundabout, eliminating any confusion about right of way. Additionally, drivers who miss their exit in a roundabout can continue around again rather than attempting a U-turn on a road they traveled down in the wrong direction. Westlandseweg’s basic, single-lane roundabout is an excellent example of how this type of intersection can move traffic, pedestrians, and cyclists in a safer and more effective manner than its 4-way signalized counterpart.

Roundabout - Basic One Lane

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Figure 1 – Westlandseweg Basic Single Lane Roundabout

As seen in Figure 1, this intersection contains the foundational elements found in nearly all roundabouts, such as horizontal deflection, a central barrier to block vision of the through lane (tree in this case), and narrow entrance lanes. These design features all help enforce the systematic safety principle of speed control, slowing vehicular traffic while not mandating a full stop. Careful consideration of the other systematic safety principles allows designers to further develop this roundabout into a pedestrian and cyclist friendly zone, as discussed later in this post.

Speed Control and Separation

Roundabouts are a crucial design feature for achieving speed control, especially with the case of thru traffic traveling on distributor roads or in residential areas.  The primary way in which roundabouts slow cars down is by requiring the horizontal displacement of a car’s path, putting forces on the vehicle’s occupants which are uncomfortable above a certain speed.  Even when a roundabout is empty and a thru travelling car can enter without stopping, this car must slow down to comfortably weave around the inner island.  Roundabouts with properly shaped curbs and splitter islands force almost a 90 degree turn for entering or exiting vehicles, which severely cuts down the speed at which cars can travel.

The separation of faster moving vehicles from slower moving bicycles is an important concept used in roundabout design whenever possible.  By providing a separated cycle track around the outside of a single lane roundabout, the interaction between cyclists and drivers is reduced to 90 degree intersections only.  Separated cycle tracks around a roundabout provide greater visibility and awareness to drivers in the roundabout of thru travelling cyclists and give them time to prevent a right turn collision.  Additionally, the splitter island included in single lane roundabouts provides separation between entering and exiting vehicles and reduces the number of lanes crossed by pedestrians or bikes on a cycle track to one at a time.


Figure 2 – Visual Depiction of Conflict Points in Roundabouts Versus Standard 4-way Stop Intersections

Predictability and Simplicity

The design of roundabouts is such that they are intuitive to any new driver and provide predictability as to how the other vehicles around will behave.  Making a left turn at a 4 way intersection requires finding a gap in traffic, assessing the speed of oncoming vehicles, and watching for crossing pedestrians or cyclists all at the same time.  A roundabout makes it so that drivers only have to focus on one of these problems at a time- yielding to pedestrians in the crosswalk then moving forward to the roundabout entrance, yielding to one lane of traffic then entering the roundabout, and yielding at a crosswalk again as the driver exits the roundabout.  Figure 2 depicts how the design of a roundabout has a much lower number of potential conflicts between vehicles and pedestrians.

Single lane roundabouts also allow drivers to know exactly what the vehicles around them will be doing because at every point in a roundabout there is a clear designation of whether a driver has the right of way or must yield.  This is further reinforced because the design of single lane roundabouts is very consistent across the Netherlands except when conditions require adaptations. One example of this is the bus and tram accommodating roundabout on Delflandplein.

Roundabout - Big

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Figure 3 – Delflandplein Transit Accommodating Roundabout

The Delflandplein roundabout is the biggest and most complex of the roundabouts discussed in this post, yet it still facilitates safe and effective travel for all users. A quick glance at Figure 3 shows a very unique feature of this roundabout: two lanes cutting through the center island. Though seemingly counterintuitive, these lanes allow for buses and trams to pass through the intersection with priority. When a tram or bus seeks passage through the intersection, vehicles currently in the roundabout are told to yield by a series of signals. This takes out any guess work for vehicle drivers, creating a simple and predictable yield scenario and allowing public transport to quickly pass through. Despite being the largest intersection in question, Delflandplein’s transit accomodating roundabout is evidence that roundabouts are capable of adapting to different situations while still maintaining their systematically safe nature.

Forgivingness and Restrictiveness

Included in the principles of systematic safety is the idea that street design should accommodate both the vulnerability of humans and the errors in judgement that we all make.  A roundabout eliminates the possibility of head-on collisions and t-bone collisions, reducing potential accidents between vehicles mainly to sideswipes which have the lowest chance of serious injury.  Furthermore, roundabouts slow traffic so that any collisions are much less likely to result in fatalities or serious injury.  Similar to the normal cross section of a Dutch street, roundabouts typically provide refuge islands so pedestrians only have to cross one lane of traffic at a time. Sidewalk curbs are also often extended at crossings in order to minimize the distance that pedestrians need to travel.

In addition to reducing the number of dangerous conflicts that can occur at a signalized intersection, a roundabout also eliminates the potential for bad behavior that we see at signalized intersections.  For instance in many cities, a green signal turning to yellow often means “speed up” to a driver, certainly not the intended consequence.  Also, right turn lanes at a high volume intersection or one with low visibility often have “No Turn On Red” signs, but these are frequently disobeyed by drivers.

State Awareness

The last principle of systematic safety, state awareness, has to do with how a cyclist or vehicle operator observes the terrain around them and interacts with it.  At a signalized intersection it is easy to zip through a yellow-turning-red signal, or to floor the gas pedal when a light turns green and quickly speed away.  In both scenarios a driver may not realize the safety risks of these actions.  Given a roundabout a driver is consciously slowing down, yielding to traffic, and navigating the roundabout such that they are more aware of their speed and their surroundings.

Examples of Dutch Roundabouts

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Figure 4 – Map of Studied Roundabout Locations

Basic Single Lane (Westlandseweg)

Roundabout - Basic One Lane

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Figure 5 – Westlandseweg Basic Single Lane Roundabout

As previously mentioned, the Westlandseweg roundabout contains the fundamental elements that make roundabouts inherently safe but careful design work takes it a step further. In line with the predictability and simplicity components of systematic safety, this roundabout includes separate zones for vehicles to yield to pedestrians and other vehicles. Both entering and exiting vehicles are able to use these designated zones to isolate the potential conflicts that they need to detect and avoid. For example, a vehicle entering the roundabout approaches the cross walk and yields to pedestrians and cyclists only. The vehicle then pulls forward into the space beyond the crosswalk and yields to vehicles already in the roundabout. This creates a simple and predictable situation where the driver only needs to make one wait-or-go decision at a time. There are also yielding zones for exiting vehicles, allowing cars to yield to crossing pedestrians while not blocking the roundabout’s travel lane.

Also in line with simplicity and predictability is the fact that all vehicular lanes intersect with pedestrian and cyclist lanes at 90 degree angles. This provides better visibility for drivers at intersections, further improving safety and predictability for all users. Crossing islands also provide refuge for pedestrians and require only one lane at a time to be crossed, simplifying the crossing process and minimizing the risk of crossing a bidirectional roadway.

Lastly, the principle of separation is present in the sense that light-weight cyclists  are removed from the motor vehicle lanes through the use of a one-way cycle tracks. There is no sense in having a bicycle weighing 15 pounds to be traveling in the same lane as a car weighing 2000 pounds. Accidents can be greatly reduced if the two conflicts are simply separated from one another.

No Pavement Markings (Hof van Delftlaan)

Roundabout - Unmarked

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Figure 6 – Hof van Delftlaan Unmarked Roundabout

A single lane roundabout may not have specific facilities for cyclists if there is not enough space at the intersection to include cycle tracks or bike lanes, and if the traffic has a low volume of speed-controlled traffic.  For the enhanced safety of single lane roundabouts, it cannot be denied that they are more space intensive than a 4-way signalized intersection (or a 4-way stop).  While it is seemingly against the principle of Dutch transportation design to not include delineated cycling facilities, there are some cases where a single lane roundabout has low enough traffic volumes that it is safe for cyclists to merge into the travel lane when using the roundabout, and then exit back onto a bike lane or cycle track.

As the single lane roundabout demonstrates, sometimes the intersecting streets meet at angles other than 90 degrees, which means that including bicycle facilities on the outer edge is even trickier.  In the case of Hof van Delftlaan, it is clear that private property extends on multiple sides to the edge of the roundabout, and it would be expensive for the city to buy off some this property and demolish existing buildings.  Because this is a low volume intersection in a residential neighborhood, it is imperative to have functional sidewalks and pedestrian crossings, but because traffic can only move at very low speeds in this area, it is acceptable for cyclists to merge directly into the travel lane of the roundabout.

Note that the sidewalks were purposely kept at a standard width around this roundabout, rather then reducing them by a couple feet to make the travel lane wider.  This was a conscious decision to keep the travel lane at a smaller radius and therefore demanding slower speeds of vehicles in the roundabout.  Because there is not enough room for a full bike lane next to the travel lane, this is the next best solution rather than forfeiting space from the pedestrian right of way.

Two-way Cycle Track (Delfgauweg)

Roundabout - Two Way Cycletrack

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Figure 7 – Delfgauweg Roundabout with Two-way Cycle Track

In addition to the foundational elements of all Dutch roundabouts such as a vision barrier, horizontal deflection, and narrow entrance lanes, Delfgauweg’s roundabout includes a less common two-way cycle track. Since this roundabout is fairly large, a two-way cycle track allows cyclists to conveniently access any leg of the roundabout, all while being safely separated from motor vehicle traffic. Other features inspired by systematic safety principles include separate pedestrian and vehicular yield zones for entering and exiting traffic, as well as 90 degree intersections between vehicles and pedestrians/cyclists, both features improving the intersection’s simplicity and predictability of use. While this intersection is incredibly effective for cars, pedestrians, and cyclists, the above video of Delfgauweg’s roundabout shows that it can also easily accommodate buses.

Half Cycle Track, Half Bike Lane (Noordweg)

Roundabout - Right Angle

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Figure 8 – Noordweg Roundabout with Mixed Cycling Infrastructure

This roundabout on Noordweg includes the previously discussed vision barrier, narrow lanes, crossing islands, and separate yield zones, but has a unique cycling infrastructure split with part cycle track and part bike lane. As seen in Figure 8, the cycling infrastructure transitions from a protected cycle track in the southern half of the roundabout to a painted bike lane in the northern half. Since the northern leg of the roundabout has bike lanes and there is not ample room on the corners for a cycle track and sidewalk, it makes sense for the bike lanes to continue into the roundabout. An important feature to note is that the bike lanes cut significantly into the North corners in order to have them intersect the vehicle lanes at the desired 90 degrees. A cyclist traveling from north to south will only have to ride in the intersection without separation for a few feet before entering a protected cycle track. Though an ideal roundabout would include purely cycle tracks due to enhanced visibility and overall safety, planners are sometimes forced to make design exceptions due to situational limitations.

Hogback Dividers (Dorpskade)

Roundabout - Hogback

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Figure 9 – Dorpskade Roundabout with Hogback Dividers

As mentioned in the Noordweg roundabout discussion, sometimes spatial limitations require creative designs in order develop a functional yet safe roundabout for all users. In the case of the Dorpskade roundabout pictured above, the available space did not permit the use of cycle tracks throughout the roundabout. Instead, designers chose to use painted bike lanes with hogback dividers, discontinuous 1-foot wide beveled curbs separating cyclists from vehicular traffic. Figure 9 illustrates the placement of the dividers in areas between vehicular entrance and exit lanes. Not only does this infrastructure help enforce the separation principle of systematic safety by strongly encouraging drivers to stay in their lane, but also provides forgivingness if a truck or plow needs to travel over the divider. Instead of hitting and likely destroying a square curb, trucks can seamlessly go over or drive on the hogback divider to complete a turn before returning to the lane. The discontinuous nature of the hogback divider makes driving on it uncomfortable to drive on, further promoting separation and correct lane usage. While this design is effective in keeping cyclists safe, the lack of cycle tracks forces cyclists’ intersections with travel lanes to be at less than 90 degree angles. This is not ideal as it negatively affects visibility but again, not all roundabouts will be able to implement all desired safety measures.

The Dorpskade roundabout is also the only 3-way intersection discussed in this post, highlighting roundabouts’ versatility to adapt to different scenarios. Lower traffic volumes at a 3-way intersection also contribute to the feasibility of using hogback dividers rather than cycle tracks. By choosing to use a roundabout rather than a standard T-intersection, the chance of head-on and T-bone collisions is entirely avoided and vehicles and cyclists are slowed rather than completely stopped. Evidently, the desirable features found in a 4-legged roundabout are just as applicable in this example.