5.5 Traffic calming

Traffic calming should be achieved through good street design. First and foremost, the intrinsic characteristics of the street should encourage slow speeds and careful, considerate driving. Where this is not feasible, such as on long continuous links, the next step should be to introduce horizontal deflections and features to reduce speeds, and lastly vertical deflections should be considered.

Intrinsic Measures.

This includes making the carriageway just as wide as it needs to be for vehicles to pass, but not wide enough for them to pass comfortably at speed. The use of on street parking creates ‘edge friction’ which helps reduce speed.

Street trees also have a similar effect, as well as improving the sense of enclosure and providing many other benefits. When setting out street trees, a spacing of between 8 and 16m is recommended, but the actual spacing should take into account parking bay and plot frontage dimensions and street lighting.

Other design features can help visually narrow the street, such as different surfacing or markings at the edges, and bringing the building line in to create greater sense of enclosure.

Reducing forward visibility is a very effective method of reducing speeds and, if feasible, the street layout should be designed accordingly with changes in direction and tight corner radii.

The use of road markings should be discouraged on secondary and Local streets to help create a less formal appearance that encourages more cautious driver behaviour.

On primary routes, designed for 30mph or under, a centre line is not required.

Following a similar rationale, fully shared surfaces are effective at regulating speed and driver behaviour but should only be used in limited circumstances, such as in mews or minor streets that do not have through vehicular traffic.

Horizontal deflections.

This includes road narrowing features, such as build outs for trees or SuDS, regular spacing of junctions and crossings, central islands and carriageway deflections to reduce forward visibility. Most of these features will be more effective with the addition of vertical deflection measures, such as raised table crossings (5.3).

When designing new carriageways, consideration should be given to bringing in the building line to create narrowing or deflections, rather that solely relying on street design features. This will be more effective and will make better use of the available space, and example of where this has been achieved is shown in Figure 5-9 below.

Vertical deflections.

Ideally vertical deflection features should be used in conjunction with other measures, such as raised table crossings and junctions.

In retrofit schemes it may be permitted to use speed humps or cushions, where other methods have failed, and provided that any ramps have a ‘sinusoidal’ profile which is more comfortable for cyclists. Such measures should be considered a last resort.

Rumble strips, usually constructed from granite setts, help provide demarcation between street types. Cyclists should be able to bypass any rumble strip, or a channel could be provided.

Spacing and service access

Traffic calming interventions, such as narrowing and raised tables, should be regularly spaced, depending on the street typology and design speed, in order to break up continuous streets that could otherwise encourage higher speeds. A maximum spacing of 70m is recommended to achieve speeds of 20mph [Reference 12].

Reference should be made to the street typology table (4.14).

When designing traffic calming measures consideration must be given on impact on refuse and emergency vehicles and the relevant teams must be consulted at the design stage.

References

• Reference 12: DfT (2007) Manual for Streets (Return to content for reference 12).