Vehicle Activated Signs

Vehicle Activated Signs (VAS) have been developed to address the problem of inappropriate speed where convention signing has not been effective.

There is an established relationship between vehicle speeds and road accidents. The latest generation of vehicle activated signs display a message (symbols and words) delineated by either fibre-optic cables or light emitting diodes (LEDs) mounted on the front panel of the sign. Different parts of the message or symbols can be shown in different colours. The sign face is provided with an automatic dimmer to reduce the intensity during night-time operation. When not activated by a vehicle, the sign remains black (i.e., blacked out).

The signs used recently have been of two types:

• Speed enforcing
• Warning of a hazard

The studies show that the signs appear to be very effective in reducing speeds and accident reduction, particularly those of the faster drivers who contribute disproportionately to the accident risk, without the need for enforcement such as safety cameras.

Vehicle activated signs are not a substitute for standard plate signing nor are they to be used as speed limit repeater signs. Vehicle activated signs will not normally target all drivers but rather that population exceeding the posted speed limit or a safe speed for the particular hazard. If used correctly they will be illuminated for only a proportion of drivers and are therefore not repeater signs.

Vehicle activated signs should be considered only when there is an accident problem associated with inappropriate speed that has not been satisfactorily remedied by standard signing and where safety cameras and related signs are not a cost effective or otherwise appropriate solution.

Inappropriate speeds might include vehicle speeds on the approach to a hazard, such as a bend or junction, that are below the posted speed limit and consequently below the police enforcement thresholds.

Before the decision to install vehicle activated signs is made, it is important to undertake an audit of existing furniture, fixed signs, road condition and road markings to assess their standard and condition.

It is not recommended that vehicle activated signs are deployed unless it is clear that the problem cannot be remedied by improving the fixed signing. It should also be noted that vehicle activated signs are not a substitute for conventional signs and they should therefore only be used sparingly.

Detailed accident investigation should also be undertaken to identify the dominant accident patterns and confirm that vehicle activated signs are an appropriate remedial measure.

Site selection should also take into consideration the number of speed-related accidents and particularly inappropriate speed for the conditions, for example, on approaches to bends and junctions. Monitoring of traffic speeds should be undertaken to establish that a problem with inappropriate speed exists.

The collection of speed data prior to the installation of the vehicle activated sign should enable the estimation of a suitable threshold speed for the sign to display the message. Speed threshold for warning signs should be set at the 50th percentile speed measured before installation. For speed limit signs the threshold should be set depending on road conditions.

It is crucial that the speed monitoring detectors are installed as accurately as possible to minimise errors in speed measurement.

The cost of running main power to the installation should be investigated at an early stage of planning in order to make contingencies for an alternative power source (solar panels/wind powered generators) if the costs are too high. However, it is important that alternative energy sources are assessed for their impact on the visual environment and increased susceptibility to vandalism and theft.

Before installing a vehicle activated warning signs, local authorities should ensure that the permanent fixed warning signs, sited in advance of the VAS, are correctly and appropriately placed. The siting distance will depend upon 85th percentile approach speed. For example, if the 85th percentile speed is 60 mph, the sign will be approximately 245 metres from the hazard. If the 85th percentile speed is 40 mph, the sign will be approximately 100 metres from the hazard.

It should be noted that placing hazard warning signs too close to the hazard could render them ineffective. The use of rectangular yellow backing boards can help make signs more conspicuous.

Once the permanent signs have been correctly installed and if the problem with the associated hazard persists, a VAS can then be considered. It is important that the VAS is also correctly sited. If it is too far from the hazard, it is possible that the association between the sign and the hazard will not be made. If it is too close, it leaves a very short time. Where signs are designed to draw attention to a hazard, they should be installed 50-100 metres in advance of that hazard, to give the driver time to respond.

Signs must not contain non-standard pictograms or messages, to avoid causing ambiguity and confusion to drivers.

The operational efficiency of VAS requires that regular maintenance is carried out. This involves cleaning the sign face, removing any obstructing foliage and in particular ensuring that the vehicle detection system is functioning correctly. It is suggested that a six-monthly inspection is carried out at all VAS locations.