Derbyshire County Council, as the local highway authority, is committed to the reduction of casualties on its highway network. There are a number of traffic calming measures available to help reduce traffic speeds, and discourage inappropriate through traffic, in order to achieve casualty reduction on our roads.
We receive many requests for traffic calming measures which far outweigh the limited funding available for such schemes. Our funds must therefore be targeted at areas with a history of speed-related collisions resulting in personal injury; prioritised to those locations with the greatest number of collisions, with pattern and severity also taken into account. Sites of concern are identified either from data analysis (speed surveys and collision history) or from members of the public, in person or via their parish council/Derbyshire County Council member. Measures can only be introduced at locations where there is an identifiable problem (i.e. trend in collisions) and will be chosen based on the likelihood of an improvement to the road safety record being achieved.
The enforcement of speed limits within the county is the responsibility of Derbyshire Constabulary and any instances of excess speeding should be reported to them, for appropriate action. You can contact your local police office by calling 101.
Below is a description of some of the speed-reduction measures we can consider, given the right circumstances. Physical calming measures - such as road humps or speed cushions (vertical deflection), build-outs and chicanes (horizontal deflection) − are costly and generally not well supported by the public and so we will tend to consider less intrusive measures wherever possible.
Perhaps the most recognisable form of traffic calming, road humps (commonly referred to as 'sleeping policemen') can be used to reduce traffic speeds and discourage inappropriate through-traffic on residential roads in order to lessen the risk of speed-related collisions occurring.
A road hump is rarely introduced in isolation and a scheme would normally include several humps, set at regular intervals, in order to reduce speeds consistently over the given route.
A variation on road humps is speed cushions. Unlike road humps, speed cushions form small plateaux across the width of the carriageway with gaps in between. Arguably not as effective as road humps, speed cushions do however allow easier passage for wider vehicles (such as those used by the emergency services) as they can straddle either side of the plateau; a useful alternative to road humps on busy bus routes and those heavily trafficked by heavy goods vehicles.
Speed tables take the form of single, raised 'table-top' plateaux across the width of the carriageway. In addition to achieving reductions in speed, tables also provide a safe crossing place for pedestrians, across the top of the plateau, where traffic speeds will be at their lowest.
Measures of horizontal deflection, as described above, can only be introduced on roads with a speed limit of 30mph or less, and where street lighting is present. We are also governed by the Highways (Road Humps) Regulations 1999 which state that humps are to:
- be between 25mm and 100mm high
- have a minimum length of 900mm
- be either curved or flat topped, and
- be spaced at between 20m and 150m.
There will need to be very clear justification on grounds of road safety for any of these measures to be introduced as they are not well supported by the general public due to their detrimental effects. These measures will invariably create a level of noise/vibration pollution for local residents. The need for associated signage and street lighting can also be considered detrimental to the aesthetic of residential areas. Given the lack of support, less intrusive measures may be more appropriate in most situations where traffic calming is required.
Build-outs, chicanes and priority narrowing
The benefit of horizontal deflection over vertical deflection is that vehicles do not have to travel over a physical feature and therefore problems of noise/vibration pollution are removed.
Such measures can often take the form of chicanes which uses features to either narrow the carriageway − allowing for two way traffic flow at slower speeds − or gives priority to drivers travelling in a certain direction, creating a break in traffic flow and reducing speeds.
Chicanes can be formed by creating footway build-outs; widening of the footway into the carriageway to provide improved visibility for pedestrians wishing to cross the road. This is of particular advantage on residential roads with high levels of parked cars. Build-outs introduced in isolation would not necessarily be used as a speed-reducing technique but the 'narrowing' of the carriageway will encourage some drivers to reduce speeds. A number of build-outs, introduced at strategic locations, will create a chicane effect and help to control traffic speeds along the route in question. Build-outs can be difficult to achieve where there are many private driveways restricting their positioning.
Priority narrowing is usually created through footway build-outs, extending into the carriageway to such a degree as to limit it to one-way traffic flow. The effect of this is that vehicles travelling in one direction have to give way to oncoming traffic, creating a break in traffic flow and subsequently reducing speeds. This measure does rely on oncoming traffic to be effective. A steady flow of traffic in either direction is needed and, if the balance is not right, can result in drivers speeding up to get through the gap first.
Footway build-outs and priority narrowing are often viewed as too intrusive by residents due to the associated kerbing required for the build-outs and signing/illumination of the priority system. An additional consequence of all forms of horizontal deflection is that it invariably removes lengths of on-street parking, which is unfavourable in areas where such provision is in high demand.
Less intrusive measures will be considered wherever possible.
As with vertical measures, horizontal measures can only be introduced on roads with a speed limit of 30mph or less, and where street lighting is present.
Vehicle activated signs (VAS)
VAS have become a popular, effective, less intrusive form of speed-reduction which can be used as an alternative to more physical measures. These are electronic signs which display a symbol and/or message when triggered by a vehicle travelling at a specific pre-set speed − the threshold speed usually being set at 10 per cent + 2mph above the posted speed limit (for example, 35mph in a 30mph limit). They are often introduced to supplement rather than replace traditional signing and lining and are aimed at addressing specific road safety problems.
Both permanent and temporary VAS measures have been utilised in Derbyshire. Research has shown that the effectiveness of permanent VAS reduces as motorists become familiar with them. The advantages of a temporary VAS is that it can be moved around between a number of sites; remaining at one site for a number of months before being moved to another site once motorists have become familiar with it. The sign can then be redeployed to the same site several months later to retain its effectiveness.
VAS have been developed in Derbyshire to address not only problems of exceeding speed limits, but also to encourage drivers to approach hazards − such as bends or junctions − at a safe speed, and to provide hazard warnings where conventional signing alone has not been effective. Analysis of existing sites has shown that, where these signs have been introduced in response to injury collision problems, they have resulted in immediate and ongoing improvements to the casualty record.
There are still relatively few signs of this nature in Derbyshire but there are concerns that to introduce them on a widespread basis would cause drivers to become used to them and their effect would diminish. In response to these concerns we apply a stringent set of criteria to each application we receive, to guard against over-proliferation and to ensure that signs are introduced where they are most needed. This allows us to determine our priorities for investment in VAS and to inform other bodies about where signs will be deployed and where installation is likely to be refused.
The protocol dictates that all of the following criteria must be met:
- VAS should be considered at sites that have a collision history associated with inappropriate speed, or a hazard, that has not been satisfactorily remedied by standard signing. Other signing means must have been tried and have failed; the site must have been subject to a recent speed survey to determine justification for a VAS installation.
- VAS displaying a speed limit should be located at sites which have a history of a minimum of six injury collisions within 1km over the previous three years, and where speed has been a factor in some, if not all the collisions.
- VAS displaying a speed limit should be located at sites where the results of traffic surveys show the 85th percentile speed is at least 10 per cent over the speed limit + 2mph, measured over a seven-day period. [The 85th percentile is the speed at which up to 85% of the traffic is travelling.]
- Hazard warning VAS should be located at sites which have a history of a minimum of six injury collisions within 1km over the previous three years, and where the hazard has been the cause.
- The flexibility of temporary VAS means they are the preferred option but the decision on which type of VAS to be used should be made on a case by case basis. To retain effectiveness, temporary VAS should remain on site for no longer than three months and should not be redeployed at the same site within six months.
Requests for VAS that meet these criteria will be prioritised on the basis of a calculated estimate of casualty reduction benefits.
Before utilising any of the above measures, we will normally consider whether road markings could be used at sites which suffer from a poor road safety record. The use of road markings can be a cost-effective measure in resolving certain speed-related injury problems.
An example of road markings we may consider are rumble strips. These would normally take the form of slightly raised strips, set across the entire width of the carriageway, and a different colour to the road surface. The strips cause vibration when driven over to alert drivers to reduce their speed and are typically used to draw attention to a change in speed limit − for example, at the entrance to villages where there have been collision problems. Due to the noise generated by rumble strips, we are not permitted to introduce them within 200 metres of residential properties.
Another technique we may adopt is visually narrowing road markings, usually taking the form of white hatching placed down the centre of the carriageway. This creates a visual effect of narrow traffic lanes, reducing speeds and keeping opposing vehicle flows away from each other. They also encourage lower speeds when overtaking cyclists or parked vehicles. 'SLOW' road markings can also be considered at problem locations.
For general enquiries on the provision of traffic calming measures please contact:
- Call Derbyshire on 01629 533190
- email: firstname.lastname@example.org