Information about the speed limit in force

Setting the appropriate speed limit is of course the first step. The second step is to assure that the driver always and everywhere knows what the speed limit in force is. The conventional way is the use of roadside signing and road markings. In-vehicle systems to inform drivers about the speed limit in force are likely to be introduced progressively.

Signing and marking

Whereas almost all drivers know what the general speed limits in their country are, there is still often uncertainty about the speed limit in force when driving at a particular road [54][56]. There are several supplementary ways to reduce the uncertainty:

Roadside signing

The conventional way to inform road users about the speed limit at a particular road or road section is roadside signing. The Vienna Convention provides guidelines for roadside signing in general, for example regarding uniformity, consistency, simplicity and legibility. With regard to speed limit signs it is important that they are placed on a regular basis; for example, a sign is usually needed after a junction. As with all other road side signs, speed limit signs need to be placed such that they are very visible. They also need to be maintained adequately. Signs may fade in sunshine or become illegible by dirt or overgrown trees.

Road markings

To support the road side signs, a speed limit sign can also be painted on the pavement, for example at speed limit transitions. Furthermore, the speed limit regime at a particular road type can be supported by differential, but consistent longitudinal lines (line present/absent, broken/solid, different colours). The meaning of the differential lines with regard to the required speed must be clearly communicated to the road users. The 'automatic' effect of longitudinal marking on speed behaviour has been found to be very small [15].

Small repeater signs as reminder

In addition to the regular speed signs, small repeater signs can help to remind the drivers of the speed limit in force. For example, in the Netherlands these small repeater signs are used at motorways that have a limit of 100 km/h instead of the general 120 km/h. These signs (with a diameter of 150 mm) are placed every 100m integrated in the hectometre posts (see photograph). In Britain, small repeater signs are required at regular intervals, where roads have speed limits which are not the commonest for that road type.

A hectometre post with speed limit reminder at motorways in the Netherlands

In-vehicle information systems

The development of in-vehicle systems to inform drivers about the speed limit in force continues rapidly. This type of in-vehicle information systems make use of detailed digital maps that are linked to a speed limit database. These systems enable the driver to get information on the speed limit in force, wherever he of she is. The EU-funded project SpeedAlert has been working on the definitions, classifications and standardisation requirements for a European application of a speed limit information and warning system (see www.ertico.com). The SafeMap project (in the framework of the French-German DEUFRAKO-programme) works on a feasibility study and a field trial with a system to inform drivers of a safe speed at a particular location. This safe speed is not necessarily the same as the speed limit. It takes account of accident numbers and/or more generic methods to assess road hazards [70].

In-vehicle information systems are actually a type of Intelligent Speed Adaptation.