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Executive Summary

A self-organizing road increases the probability that a driver will automatically select appropriate speed or steering behavior for the roadway without depending on road signs. The geometric features of the road encourage the desired driver behavior, and do not rely on the driver's ability or willingness to read and obey road signs. A perfect self-organizing road would not require speed limit signs and curve advisory warnings.

While the United States has some examples of self-organizing roads, such as using curved road segments in national parks to limit driver speed, this concept is far more common in Europe. It is easy to understand that geographic topography can create a self-organizing road that limits driver speed selection (figure 4). It is harder to appreciate intentionally designing a road to be self-organizing in an urban area.

Figure 4. Topography can create self-organizing roads. (SINTEF)
Topography can create self-organizing roads

A roundabout is a self-organizing road (figure 5). The road geometry forces the driver to select a lower speed than used on a tangent. This reduced speed design through the intersection improves safety by reducing vehicle energy when crashes occur, but also by the types of crashes that do occur (sideswipes versus head-on and crossing). The design also often improves mobility and reduces congestion because of the reduced waiting times at a signal. Pavement markings reinforce and help the driver perceive this lower speed requirement.

Figure 5. A roundabout is a self-organizing road. (SINTEF)
A roundabout is a self-organizing road

In a similar manner, intentionally narrowing the roadway and shoulders also creates self-organizing features that instruct the driver to slow down. When a conflict exists between road features and signs, drivers often follow the speed implied by the roadway design rather than the speed instructed by the road sign. For example, building a connecting roadway to interstate design standards and putting a 30 mi/h (50 km/h) sign on the side of the road would encourage drivers to ignore the speed limit displayed on the sign.

Figure 6. Scan team members examining 2+1 road in Sweden.
Scan Team members examining 2+1 road in Sweden

2+1 Roadway Design

Another important example of a self-organizing road is the 2+1 roadway design the team observed in Finland and Sweden. This road design also offers significant safety advantages, especially with the cable barrier in a flush divider used in Sweden. The road is designed to eliminate the risk of head-on collisions. As shown in figures 7 and 8, the 2+1 roadway is a three-lane road with the passing lane alternating on each side of the road in a regular manner. This organizes the driver's expectations about being able to pass.

One of the team's most impressive observations involved watching Swedish drivers approaching the end of a passing lane. During a 20-minute observation interval, no driver speeded up to pass a slower vehicle before the passing lane ended. Passing slower vehicles in advance of lane drops is common driver behavior in the United States. The expectations induced by the 2+1 design reassured drivers through the use of effective signing that another passing opportunity would occur shortly. Hence, drivers did not feel a need to pass immediately and so did not incur risk by trying to pass just before the passing lane ended.

Even in more congested conditions, traffic flow remained stable, as passing was reduced and drivers maintained more uniform speeds. Early skeptics, such as emergency responders who expected additional delays in going around median cable guardrail to get to crashes, became highly supportive of the 2+1 design because of the vast reduction in crashes they needed to respond to and the ease of removing the cable barrier when necessary.

Swedish experience with this design has been better than expected. Level of service has been equal or better at directional flows of up to 1,400 vehicles per hour, with a capacity of 1,500 to 1,600 vehicles per hour in one direction, some 300 vehicles per hour fewer than for an ordinary two-lane, 13-m (14.2-yd) road. Traffic safety effects also have been better than expected. By June 2004, there had been nine fatalities, compared to the normal 60, and an estimated 50 percent reduction in severe injuries. Chapter Five of this report discusses speed management, which can be greatly influenced by roadway design. Higher standards for design tend to promote higher speeds.

Median cable barrier crashes are very frequent, but normally without personal injuries. Crashes are often caused by skidding, flat tires, or loss of control of the vehicle. Maintenance problems are fewer than expected, but barrier repairs are major concerns. Maintenance costs have increased almost 100 percent per year, although 70 percent of barrier and car repair costs are paid by insurance companies.

Figure 7. 2+1 roadway. Note the cable barrier and sign indicating the length of the passing lane. (Sweden)
Note the cable barrier and sign indicating the length of the passing lane

Figure 8. 2+1 cable barrier. The post is easily removed for maintenance. (Sweden)
The cable barrier

Figure 9. Proposed standard 2+1-cable barrier cross section within existing 13-m (14.2-yd) roadway. (SNRA)
Proposed standard 2+1 cable barrier cross section within existing 13-m roadway

Urban Design

The concept of self-organizing roads applies equally to urban roads, where often the design goal is to induce drivers to maintain lower speeds. Streets and roads must be built according to desired functions and driver behaviors. For example, wide arterials encourage higher speeds. Lower speeds can be encouraged on local streets (e.g., at pedestrian crossings, school zones, etc.) by narrowing the road. Figure 10 shows how constricting a local road with barriers, signs, and pavement markings induces the driver to slow down. While traffic-calming techniques are widely used in the United States, European urban road designers sometimes apply such techniques more severely, even to the point of temporarily eliminating one lane. For example, in Europe it is common for a two-lane street to be narrowed for a short segment to a single lane. This compels drivers to either stop or slow down because of the possibility of oncoming traffic in the opposite direction (figure 10).

Figure 10. Example of road narrowing in the Netherlands.
Example of road narrowing in the Netherlands

2+1 Roadway Design Contact Information

Mats Petersson
Swedish National Road Administration
SE-551 91 Jönköping
Phone: +46 36 19 20 18

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Page last modified on November 7, 2014.
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