Traffic Calming Measures

Most traffic calming measures are made up of a selection of one or more features. These include:

• Gateways
• Signs, lines, markings
• Coloured surfacing
• Textured surfacing
• Changes to speed limits
• Safety cameras
• Reduction in carriageway width
• Traffic regulation measures including prohibition or restriction of selected categories of traffic
• Changes to junction priorities
• Horizontal deflection
• Vertical deflection

Road Humps

The generic term ‘Road Hump’ is used to describe a range of measures which are legal obstructions on the surface of highway, constructed to control the speed of vehicles in build-up areas.

Road humps are constructed in a variety of shapes and sizes to cater to different situations and locations and a traffic calming schemes could include a combination of different types of humps. However, a mix of round and flat top humps could create difficulties for drivers, due to different acceptable speeds of approach to each types of hump.

Road humps can only be used on roads with a speed limit of 30 mph or less.

Types of humps currently in use include:

• Normal flat top humps
• Speed tables (long flat top humps)
• Junctions plateaus (raised at junctions)
• ‘H’ humps
• ‘S’ humps
• Sinusoidal humps (may be round or flat top)
• Thumps
• Mechanical humps

These measures all consist of a physical raising of the carriageway surface to cause a vertical deflection (or change of level) for passing vehicles. Each type of hump has different qualities, benefits and dis-benefits, so the choice of traffic calming features and its actual design must be made carefully.

Round Top Humps

These are a low cost highly effective method of speed control. Historically, the ‘round top’ hump has been the most widely used form of road hump and is generally more effective in reducing speeds than various other means that have been tried. It does, however has the disadvantage of causing discomfort to vehicle occupants as well as potential noise disturbance to nearby residents.

Road top humps can produce serious noise and vibrations problems caused by passing HGVs and are more suitable for residential roads with new HGVs, buses and emergency services vehicles. However, in recent years the unpopularity of road hums has led to the increasing use of other type of calming devices.

Flat top humps are often used in streets where there is a high frequency of conflicting movements between vulnerable road users and motorised vehicles, such as in residential and shopping areas. They are very effective at providing smooth and easy crossing places for pedestrians. Combined with the provision of central islands, this can change the overall look of an area to being much more pedestrians friendly and less like a main route for vehicle use only.

‘H’ and ‘S’ shaped, flat top humps are a variation of flat top humps designed to reduce the physical impact on buses and emergency services, while remaining effective in controlling the speeds of smaller vehicles.

Flat Top Humps/Speed Tables

The use of ‘flat top’ humps has a less severe impact on traffic than round up humps but will enable pedestrians to cross the road more easily. There are a number of different profiles in use and each will affect traffic speed in different ways, depending on the height, shape and length of the hump as well as the speed characteristics of the road. The flat top road hump creates a surface that is level with the footway and can incorporate a zebra or signal controlled crossing, as well as constraining vehicle speed at that point. Care should be taken when constructing kerb to kerb flat top humps to ensure they do not exceed the maximum regulated height of 100mm.

Speed Cushions

The speed cushion was initially developed to enable vehicles with a wider track width (such as emergency service and public service vehicles) to traverse it with less vertical deflection than narrower private cars. A speed cushion is essentially a short, narrow, flat top hump that has to be replicated across the full width of the road, to prevent drivers taking dangerous avoiding actions by travelling on the opposite sides of the carriageway into on-coming traffic.

The size and shape of speed cushion needs to take account of the target traffic speed. The layout of cushion is also crucial, particularly where there is on-street parking.

On routes used by emergency services it is advisable to use cushions no wider than 1600mm. The height should be no greater than 75m and width between cushions or cushion to kerb should not generally exceed 1000m with 120mm as the maximum and 750mm as the minimum. The longitudinal spacing between sets of cushions should normally be 75mm maximum, less in 20mph zone/area.

Cushion can be invisible to pedestrians crossing the road and cause them to trip, so it is useful to mark them with white thermoplastic edge markings.

Construction of Humps

Typically, road humps implemented for permanent use are constructed from flexible bituminous material. A fairly dense mix (e.g. HRA) gives stability against deformation. Pre-formed units in concrete or rubber are also available and require positive fixing to the carriageway usually by means of bolts. However even when fixed to a concrete carriageway, these materials can be susceptible to movement over time.

Humps constructed from flexible bituminous surfacing have proved to be the most resistant to deformation or deterioration under traffic loads. The use of block paving (or other small element paving) on ramp can result in high maintenance costs and are potentially dangerous surface where paving has worked loose. In some circumstances however the use of coloured surfacing and specialist material can add to the visual impact of the feature and contribute to the visual qualify of street scene.

When humps are added to an existing road surface, tapered edges can be used to leave narrow drainage channels, thus avoiding expensive gully alterations. However, this will restrict easy use by pedestrians, especially those with prams or wheelchairs, wanting to cross the roads. On newly constructed highways drainage design can be incorporated with the proposed hump in which case humps can be built full height to the edge of the kerb to provide a continuously level footways, except where this would lead to a hump height of more than 100mm.

Road Narrowing, Footway Buildouts and Chicanes

Reducing the available road space for drivers by constructing road narrowing features alters the appearance of the streets and has the effect of lowering the speed at which drivers feel safe to drive. Road narrowing is form of horizontal traffic calming, as distinct from the vertical traffic calming.

The reallocation of street space can improve the quality of life of residents and reduce the dominance of motor vehicles.

Carriageway narrowing can take the form of:

• A gateway feature which retains two-way flow of traffic.
• A pinch point where traffic can only pass through the feature in one direction at a time (known as priority one-way working).
• Central islands to prevent overtaking
• Chicanes to force traffic to deviate from straight through path
• Reduced width over a length of road (while still being suitable for two say HGV traffic)
• Reduced width over a length of road within the use of advisory cycle lanes to visually narrow the street.
• Reduced width over a length of road such that light vehicles can pass each other but larger vehicle have to give way.

A combination of one or more of the above features can be used and overrun areas can also be provided to allow longer vehicles to pass whilst retaining the visual effect of a narrow street.

Risks to cyclist increase, if the lane width is insufficiently narrow to prevent overtaking or wide enough to allow it to occur safely.

Traffic islands must be clearly visible to approaching drivers at all times and any street furniture should be set back at least 0.5m from the kerb edge. A series of island will prevent overtaking when it is felt to be dangerous.

Soft landscaping can be attractive option for traffic islands, but safety problems can arise if unsuitable planting is used (quick and high growth) or if there is insufficient maintenance in future years.

Concern has been expressed by some safety auditors about the use of footway buildouts for the introduction of priority one-way working, where these features are used to give pedestrians a shorter route across the carriageway. They consider that drivers may be distracted from watching for pedestrians because of the complexity of the road layout. This is also the case where islands create a horizontal deflection, so careful consideration needs to be given to these issues during the design process.

Conspicuity of features built within the carriageway is important.

Where narrowing or footway buildouts are constructed to form chicanes, the necessity for the design to accommodate larger vehicles may reduce the effectiveness of the scheme. Conversely layouts which are too confined can lead to drivers overrunning the kerbs.

Designers should also be aware of the potential for some drivers to race to get through chicanes or build-out ahead of oncoming traffic, which may have the signed priority.