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Visibility in Highway Design – DMRB & Manual for Street

March 13, 2024

Visibility in Highway Design – DMRB & Manual for Street

Visibility is a cornerstone of highway safety, profoundly impacting driver decisions and response times. Insufficient visibility can precipitate accidents, particularly at junctions, crossings, and bends where clear sightlines are indispensable for anticipating hazards and navigating safely. Optimal visibility not only enhances safety but also enhances the comfort and efficiency of road users, facilitating smoother traffic flow and mitigating congestion. In the United Kingdom (UK), comprehensive guidance documents govern highway design, prominently among them the Design Manual for Roads and Bridges (DMRB) and the Manual for Streets (MfS).

The choice between these guidelines typically hinges on road speed. The DMRB guidelines are apt for high-speed roads with an 85th percentile speed exceeding 60 km/h (37mph), while MfS is better suited for roads with an 85th percentile speed below this threshold.

Whilst speed limits of roads give a good approximation of likely vehicle speeds on a given stretch, actual vehicle speeds can be much faster or slower dependent on the character of the road (windy, open, poor surfaced etc) leading to different driver perceptions and speed responses. To this end, actual vehicle speeds for design purposes should be determined via a vehicle speed survey to ascertain the 85th percentile speed.

The key parameter which secures an adequate visibility is the Stop Sight Distance (SSD). The SSD is the minimum distance required for a driver to perceive an object or hazard on the road, apply the brakes, and come to a complete stop safely. This concept is crucial for ensuring that drivers have adequate visibility to react to potential hazards, such as vehicles ahead, pedestrians crossing the road, or obstructions in the roadway. Below are some extracts from DMRB & MfS which set the required SSD based on the road speed.

Figure 1 – Table 7.1 Derived SSDs for streets from Manual for Streets.

 

Figure 2 – Table 2 from DMRB Volume 6, Section 1, Part 1 – TD9/93.

Figure 3 – Table 3 from DMRB Volume 6, Section 1, Part 1 – TD9/93.

The Stop Sight distance is used to determine the following key parameters in highway design:

  • Forward visibility: This refers to the distance a driver must see ahead to halt safely for obstructions on the road. The minimum forward visibility required equates to the minimum SSD. It is measured along the centerline of the inner traffic lane between points on a curve

Figure 4 – Figure 7.19 from Manual for Streets.

  • Visibility splay at a junctions: This concept addresses the unobstructed views accessible to drivers at intersections or junctions, ensuring adequate inter-visibility between vehicles on major and minor arms. It is defined by two parameters: X, the distance back along the minor arm (typically 2.4m), and Y, based on the SSD.

Figure 5 – Figure 7.18 from Manual for Streets.

 

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Visibility in Highway Design – DMRB & Manual for Street

March 13, 2024

Visibility in Highway Design – DMRB & Manual for Street

Visibility is a cornerstone of highway safety, profoundly impacting driver decisions and response times. Two main guidance documents govern highway design – DMRB and Manual for street.