Traffic conditions

An increasing number of countries monitor traffic flow and use this information to inform through matrix signs drivers about (the chance of) congestion. This application is generally restricted to motorways and some of the most important rural roads. The information may consist of a general message, that congestion is ahead or may arise, to advisory reduced speed limits and compulsory reduced speed limits.

A safe speed limit: road function and design speed

The speed limit needs to reflect the safe speed. Whether a speed is safe depends on the function of the road and, related to this the composition of the traffic flow (e.g. mixture of pedestrians and motorized traffic);

the characteristics of the traffic situation (e.g. the density of at-grade intersections).

It also depends on the road design characteristics related to design speed, such as

horizontal alignment (e.g. road width, obstacle free zones);

vertical alignment (e.g. type of curves, gradients, 'design consistency').

Matching speed limits to human injury tolerance in different potential impacts

In Sweden, the concept of a safe speed, as originally discussed by Tingvall and Haworth (1999), has been adopted as a basis for considering appropriate speed limits. The driver/vehicle/road system should operate such that, in the event of an impact, forces are not exerted on vehicle occupants or other road users which are likely to lead to a fatality. Thus, where pedestrians are present, vehicle speeds should be no higher than 30km/h. Where vehicle to vehicle impacts occur they should be at speeds below the impact speeds at which cars can be shown (through the European New Car Assessment Programme) to safeguard occupant life. Ratings are being developed through the European Road Assessment Programme showing how well the road is designed to ensure forces involved in impact with road infrastructure also keep within the same thresholds, and these are being used in Sweden to indicate appropriate speed limits for roads with different ratings.

Road type/traffic situation	Safe speed (km/h)
Based on Tingvall & Haworth (1999) the updated Dutch Sustainable Safety philosophy presents the following requirements with regard to maximum speeds in different traffic situations
Roads with potential conflicts between cars and unprotected road users	30
Intersections with potential side impacts between cars	50
Roads with potential head-on conflicts between cars	70
Roads where head-on and side impacts with other road users are impossible	≥100

Unfortunately there is not yet sufficient knowledge to define the safe speeds for motorized two-wheelers and heavy good vehicles. Also from a practical point of view this problem is as yet unsolved. The best solution is the separation from other traffic, but it is not clear how to realise that in practice."

Source: Wegman & Aarts, 2005 (page 14; translated from Dutch)

Speed limits and road function

Ideally, a road network consists of a limited number of monofunctional roads. For example, in the Netherlands, Sustainable Safety distinguishes between three road functions.

Flow function:

Roads with a flow function allow efficient throughput of (long distance) motorized traffic. All motorways and express roads as well as some urban ring roads have a flow function. The number of access and exit points is limited.

Area distributor function:

Roads with an area distributor function allow entering and leaving residential areas, recreational areas, industrial zones, and rural settlements with scattered destinations. Intersections are for traffic exchange (allowing changes in direction etc.); road links facilitate traffic in flowing.

Access function:

Roads with an access function allow actual access to properties alongside a road or street. Both intersections and road links are for traffic exchange.

At roads with a flow function and at the links of roads with a distributor function speeds of motorized traffic can be allowed to be high if

Motorized traffic is physically separated from pedestrians, cyclists, mopeds and slow moving agricultural vehicles; and
Road design standards are good

At roads with an access function and at intersections of roads with a distributor function speed must be low since here all road users make use of the same space. At these locations road engineering measures may be required to support the low speed requirement.

Speed limits and design speed

In general terms, the design speed of a road can be defined as the highest speed that can be maintained safely and comfortably when traffic is light [21]. More specifically the design speed is used by road engineers to determine the various geometric design features of the roadway [2][25]. The exact definition differs from country to country.

In principle, the required design speed depends on the function of the road and, hence, on the desired speed level. If, because of the road function, high speeds are desired, road quality and roadside protection need to be of an appropriate standard. The alternative to improving road standard is to reduce the speed limit consistent with the standard and risk of the road. The exact values for design standards of different road types differ as well from one country to another.

Clearly, the design speed must never be lower than the speed limit. It is not wise to have a speed limit which is much lower than the design speed of a road. This may damage the credibility of a speed limit.

Furthermore, it is important that the design speed is consistent over a longer stretch of road. A substantial reduction of design speed at a particular site must be supported by more than just a sign with the reduced speed limit. Additional warning signs should preferably be accompanied by a change in road design characteristics and/or road markings.

Credible speed limits: characteristics of the road and road Environment

A credible speed limit is a limit that is considered to be logical by (the majority of) drivers for that particular road in that particular road environment. It is incredible when, for example,

The speed limit sign for built-up areas is located 'in the middle of nowhere' when actual buildings and town activities are not yet visible
The same speed limit is applied for a wide, straight rural road and a narrow, winding rural road
If different limits are applied for motorway sections with a similar cross section and a similar (rural) environment. (If other reasons than safety are the basis of these different limits, e.g. noise protection, environmental pollution, this must be clearly communicated to the road users (compare the German sign 'Lärmschutz', i.e. noise protection)

In general, the principle of credibility implies that any transition from one speed limit to another must be accompanied by a change in the road or road environment characteristics.

Credibility of speed limits can be further enhanced by applying different speed limits for different weather and traffic conditions, i.e. by a system of dynamic speed limits.