Roads should be designed to cater for a defined function. This typically reflects the distance of travel, level of traffic flow and desired speed of travel. Road networks in most countries will therefore reflect the development of a hierarchy of roads, with motorways at the highest level and local access roads at the lowest. In practice a basic hierarchy will occur naturally through the more heavily trafficked roads being engineered to higher standards. But it is important that the hierarchy is established to clear guidelines linking design to function, throughout the network. This is particularly necessary where different functional levels or different geographical areas are managed by different road authorities.
It is well established that managing roads and traffic to safety management principles in urban areas can produce overall accident reductions of at least 15% . There are also good examples of integrating safety management with other urban planning objectives . Similarly it is well established that most serious injury accidents on rural roads are associated with a small number of accident types which can be addressed by different aspects of engineering design . These accident types occur in different patterns on roads with different designs and speed limits .
At the simplest level, road function can be divided into three groups - arterial or through traffic flow routes, distributor roads, and access roads. These can be defined as .
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. Junctions are for traffic exchange (allowing changes in direction etc.); road sections between junctions should facilitate traffic in flowing.
Roads with an access function allow actual access to properties alongside a road or street. Both junctions and the road sections between them are for traffic exchange.
The first two of these groups may be further subdivided into primary and local arterials and distributors, reflecting different flow levels within each group.
Roads are also often grouped by design “types”, i.e. motorways, other divided roads, 2-lane roads. Whilst motorways will always cater for a flow function, the other road types are often not used consistently to reflect a particular function, and designs within the road type groups can vary considerably. On average there are large differences in accident rate (both per km and per vehicle km) on the different road types , and thus national accident rates can be reduced by ensuring drivers use the most appropriate road for their trip purpose, and that the road design is optimized for its function.
The match between driver behaviour and road design will be optimized where the road design gives a clear message to road user of the function of the road, and the hazards that are likely to be encountered.
High proportion of traffic on motorways in the Netherlands influences fatality rates
An intensive period of motorway building during 1970s and 1980s in the Netherlands has resulted in the 40% of the national total of vehicle kms being travelled on motorways, which have low fatality rates compared with other rural roads, compared with 20% in Britain and 14% in Sweden. This high usage is encouraged by both the high density of the motorway network and the high population density. The density of motorway network in the Netherlands is four times that in Britain and 18 times that in Sweden. Population density in the Netherlands is, on average, about 60% higher than in Britain and almost 20 times that in Sweden.
Source: Koornstra et al (2002)
Risk on any specific road can be defined in terms of risk to each individual driver using the road (accidents per vehicle km) or collective risk of all drivers using the road (risk per km). High flow roads will have low individual risks but high collective risk. Investment to reduce accidents on high flow roads is more likely to be justified than investment on low flow roads because a larger number of drivers benefit. Investment in accident reduction is still likely to be worthwhile on those low flow roads where individual risk is significantly higher than average for these roads.
Current accident databases reflect the road classifications used by the accident record forms in each country. International databases, such as IRTAD, provide comparable data on more generic road type groupings (motorways, A Class non urban roads, etc), but the design of roads within these groups varies between countries.