Road classification

Authored by: Keith M. Wolhuter

Geometric Design of Roads Handbook

Print publication date:  April  2015
Online publication date:  October  2015

Print ISBN: 9780415521727
eBook ISBN: 9781482288728
Adobe ISBN:




All roads have in common that they are owned by someone, usually a statutory body, are paid for by direct or indirect taxation or tolling and form part of a network. In any form of engineering endeavour, structure is important, and this is addressed through various forms of classification.

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Road classification

3.1  Introduction

All roads have in common that they are owned by someone, usually a statutory body, are paid for by direct or indirect taxation or tolling and form part of a network. In any form of engineering endeavour, structure is important, and this is addressed through various forms of classification.

The needs for any system of road classification vary, and these are discussed in the following sections. Classification can be based on

  • Road status
  • Route numbering as a guide to funding
  • Route numbering as an aid to navigation and tourism
  • Function

with the last mentioned being that usually applied to geometric design.

3.2  Road Status

Each road authority has its own unique numbering system, usually based on the perceived relative importance of each road in the authority’s network. The road authority may refer to roads as being primary, secondary or tertiary, or main, divisional, district or county roads or any other grouping of nomenclatures.

This serves as a guide to various departmental functions such as allocation of maintenance effort or to the splitting of finances between departments and others such as local rate payers. For example, on roads that are very low in the hierarchy of the rural network, the policy may be that the department send a grader out at regular intervals to blade the road, with any other work being undertaken or funded by local farmers. The latter work could include keeping the verges tidy, clearing drainage channels, and so forth.

3.3  Route Numbering as a Guide to Funding

In many countries, numbering defines the allocation of funding, whether at the national, state or province or local level. In Britain, numbered classification was originally used for the purpose of keeping track of these funding allocations. The country is divided into nine zones and, within each zone, roads are classified as either A or B routes. The major roads, or A routes, radiating from London are all given single-digit numbers, with the digits increasing in a clockwise direction. The A1 is from London to Edinburgh.

Reference is made in various countries to numbered trunk roads. These are roads connecting major centres and are usually the recommended route for long-distance and freight traffic. In the United States, the interstate highways would be the equivalent of trunk roads. The 14 member states of the Southern African Development Community have jointly created a network of numbered trunk roads serving freight movement and with a common set of minimum geometric standards.

3.4  Tourism Routes

Another form of classification is the numbering of routes from end to end whether or not the various segments of the route have any other number derived from an administrative classification system. This numbering of routes is intended to serve the needs of tourists. Dependent on the extent to which they cross borders between the various authorities, numbered routes such as tourism networks may fall within the remit of any one of them or be a shared responsibility. To this end, road maps show the major routes not only by the convention of line style and colour but also by number. This applies particularly to rural areas and would focus on major rural routes.

Within urban areas, numbering of routes is a convenient way of defining roads that would at least have the status of being collector roads, as discussed further in the following sections.

3.5  Administrative Classification

Administrative classification addresses the typical hierarchy of government agencies, from central government down to the lowest level of local government. In general, this hierarchy comprises three levels: the national level, the provincial or state level and the local level, which, in turn, may comprise various sub-levels such as county, city, town, village or rural area.

The administrative classification of roads usually has a bearing on the method of financing their construction and maintenance apart from the obvious distinction being drawn between the various tiers of government.

Funding may be generated by fuel levies and these would go directly to the national or central level of government. The lower levels of government would usually receive a grant of funding from this source. In addition, on the basis of its importance to some or other national imperative, the central government may elect to subsidise or bear the total cost of whatever project is being mooted.

3.6  Functional Classification

Road classification implicitly acknowledges that the roads network has a built-in hierarchy of roads based on their functions and capacities. The basic hierarchy comprises arterials, collectors and local roads. Freeways are essentially arterial routes but their operation is different from that of all other roads. For this reason they are usually regarded as a separate hierarchical group.

For geometric design, the most useful form of classification is functional classification, as it defines the spectrum of road usage from pure mobility to pure accessibility. This, in turn, supports the selection of the design speed and the design vehicle. These two parameters, in combination with current and anticipated traffic volumes, define geometric standards of the horizontal and vertical alignment, and intersections or interchanges and definition of the cross-section.

3.6.1  Trip component

A trip may have only a single component, such as passenger car from home to the home of a friend. Most trips, however, comprise several components. A commuter could leave home on a local street and then move to a collector that connects via a transition ramp to a freeway or other arterial. After the relatively long-distance high-speed main movement, the trip has the same elements as those that preceded the main movement but in reverse order. An exit ramp provides a transition to a distributor that may be a moderate speed arterial taking the commuter closer to the intended destination. The arterial would then be left and a short journey on a collector would end at a parking facility that may be on- or off-street parking. This hypothetical trip comprises 10 stages.

It is not necessary for a journey to go through all the elements of this hypothetical trip and some can be left out. As a general rule, though, there should not be gaps that leapfrog more than one of the elements. This is because all the elements of the trip will still be there but will take place on some substitute for the missing element. An off-ramp may connect directly to a parking garage but would join a distribution road within the garage connecting to the various parking areas. Collector roads or lanes would provide a link to the parking bays. The parking aisle within the parking bay would take the vehicle to the individual parking spaces. In short, all the elements of the trip are identifiable. And the roads that are the network equivalents have been highlighted.

It would not be wise for an off-ramp to connect directly to a local street, for example. Two possible inferences can be drawn from this omission.

  1. The local street cannot accommodate the flow discharging from the freeway, resulting in a backup onto the mainline lanes of the freeway. Vehicles travelling at a speed of 120 km/h or more may unexpectedly be confronted by a queue of stationary vehicles with a high probability of a multivehicle crash.
  2. The flow on the freeway is such that the local street can accommodate it. The turbulence of vehicles diverging from the freeway may have an adverse effect on the mainline lanes, particularly through the lane changing that precedes the departing vehicle arrival in the outside lane prior to the diverging manoeuvre. The level of service on the mainline could thus drop to an unacceptable level, resulting in upstream congestion.

The only conclusion that can be drawn is that the trip components will all be present, whether or not the required network elements are present. Provision must therefore be made for them.

3.6.2  Mobility versus accessibility

The example of the elements of a trip as shown previously implies that they happen at different volumes and speeds. At the one end of the scale, the local street is geared to the provision of access to individual properties or other trip ends such as parking garages. The major part of the trip is taken at high speeds over significant distances so that design for mobility becomes the keynote. Unfortunately, these are not the opposite sides of a coin although they are linked in the sense that accessibility is becoming appreciated as a co-equal goal with mobility. It is axiomatic that travel is a derived demand. In short, except when on holiday, people don’t normally travel for enjoyment but rather to reach some opportunity available at a destination. This has an important implication in that the traditional assessment of success in transportation being an emphasis on mobility is, in fact, incorrect.

Transportation planners and engineers generally attach significant value to the alleviation of roadway congestion, that is, increasing mobility. If mobility is achieved through adding infrastructural capacity to the network, the outcome could be that desired destinations could be located at greater distances apart. For example, in times gone by, rural villages tended to be spaced at 50 kilometres. Personal transport was by horseback and the farmer located halfway between villages had a travel time of about 1 hour. With personal transport now being by passenger car, a travel time of 1 hour equates to a distance of about 100 kilometres, suggesting that towns located at a spacing of about 200 kilometres would flourish and those in between would tend to decline.

Increased mobility requires more time and money spent on travelling, hence less time and money to spend at the destination. In terms of the concept of the derived demand, people don’t necessarily wish to travel as such; they only wish to arrive at their destination. The increased cost in terms of time and money of reaching a destination is thus counter-productive (Levine and Garb, 2002).

Relief from congestion should thus be rejected as an independent goal of transportation policy. Enhanced mobility can be valued only insofar as it enhances accessibility whereas mobility gains resulting in a loss of accessibility represent a failure of transport policy. If destinations are fixed in space, an increase in mobility results in cost per kilometre being reduced – an increase in accessibility. If an increase in mobility makes it possible to live further away from the place of employment, mobility gains result in a loss of accessibility. The emphasis of the focus on mobility when accessibility is perceived as some sort of poor relation should thus be replaced by accessibility as being the prime measure of transportation efficiency.

The proportion of service shown in Figure 3.1, which has been widely used for several years as an illustration of the continuum from pure mobility to pure accessibility, should thus be scrapped as being misleading.

It would appear that congestion is not necessarily bad per se. It becomes bad only when it increases the time and money spent on accessing a destination. In fact, strategies aimed at reducing congestion could be inimical to accessibility by causing travel distances to grow and land uses to spread, as discussed earlier. In contrast, congestion pricing could be designed to promote more appropriate pricing of scarce roadway space and facilitate the alternative to a growth in travel distances.

This does not suggest that congestion pricing should actually be implemented to keep vehicles away from congested areas or propose the levying of a charge for entering an already congested area, as has already been applied by various metropolitan authorities such as that of London. It is rather intended to identify the effect of a proposed project on accessibility overall rather than simply testing its ability to improve capacity and hence mobility (Levine and Garb, 2002).

Mobility versus accessibility.

Figure 3.1   Mobility versus accessibility.

(From American Association of State Highway and Transportation Officials. A policy on the geometric design of highways and streets. Washington, DC, 2011a.)

3.6.3  Definition of urban and rural areas

Everyone seems to know the difference between urban and rural areas on the basis of, ‘If you’re surrounding by buildings you’re in town. If you are surrounded by farm lands, you’re in the country’. If it only were so easy. Most countries operate on the basis of the definition of urban areas, with the rural areas being anything that is not classified as urban.

In the United States, urban areas are those with a population of more than 5000 people. There are two categories of urban areas differentiated on the basis of total population. ‘Urbanised areas’ are those with a population larger than 50,000 and ‘small urban areas’ have populations of between 5000 and 50,000. According to the Census Bureau, urban areas are those with a population density of 386 people per square kilometre (1000 people per square mile) in the core area and 193 people per square kilometre (500 people per square mile) in the surrounding area. In the U.S. Department of Agriculture’s natural resources inventory, urban areas are officially known as developed or built-up areas of more than 4 ha (10 acres).

In Canada, urban areas are designated as ‘population centres’ if they have a population of at least 1000 people with a density of 400 or more per square kilometre.

In its Road Traffic Act (Act 29 of 1989), South Africa defines an urban area as that falling under the jurisdiction of a municipal or metropolitan authority and that has been subdivided into individual properties including the roads abutting on them as listed on the municipal cadastral map or cadastre, with the latter providing much more detail than the map. The municipal commonage thus falls outside the urban area.

In the United Kingdom, the definition is based on the extent of ‘irreversible urban development’ indicated on ordnance survey maps. The definition is an extent of at least 20 ha and at least 1500 census residents. Separate areas are linked if less than 200 metres (220 yards) apart. If the urban area includes an Anglican cathedral, it acquires the status of a city.

3.6.4  Functional network components of rural areas

Rural networks comprise

  • Arterial roads
    • Major arterial roads
    • Minor arterial roads
  • Collector roads
    • Major collector roads
    • Minor collector roads
  • Local roads

The principal arterial system accommodates long-distance travel and movement between all urban areas with populations of 50,000 people and most of those with populations of more than 25,000 people. These roads are usually freeways. Minor arterials provide linkages between cities and large towns, including other traffic generators such as major resort areas. The spacing of minor arterials is such that all developed areas are within reasonable distances of major arterials. Trip lengths and travel densities are greater than those of local roads but less than those of major arterials.

Rural collectors serve trips with lengths shorter than those served by arterials at lower speeds than those typically encountered on arterials. Major collectors provide linkages to towns not directly served by arterials, linking them also to larger towns or cities or to arterials, both major and minor. Minor collectors are spaced at intervals consistent with population densities to provide linkage to higher order roads in the network. They also serve smaller communities and provide a service to rural areas by linking them to locally important centres.

The primary function of local roads is to provide access to individual properties adjacent to the network. Travel on local roads is typically over short distances at relatively low speeds. Local roads are all those not classified as arterials or collectors.

3.6.5  Functional network components of urban areas

The components of an urban network are

  • Arterial roads
    • Major arterial roads
    • Minor arterial roads
  • Urban collector roads
  • Urban local streets

The major arterial roads are the effective backbone of any urban system. They carry large volumes of traffic at high speeds from one end of the urban network to the other. They thus perform a valuable service to through travel and rurally oriented traffic. Their importance also derives in large measure from service to major circulation movements within the urban area. Major arterial roads are usually freeways with complete control of access by the use of interchanges. As a general rule, the minimum spacing between successive major interchanges can be as shown in Table 3.1.

The spacing between parallel freeways is addressed in Chapter 11. The concept of the white line breakpoint distance, which is suggested as the form of measurement of interchange spacing, is introduced in Figure 11.6.

Table 3.1   Spacing of interchanges


Urban areas

Rural areas

Access to access

1300 m

2170 m

Access to systems

2100 m

3270 m

Systems to access

1420 m

2170 m

Source: Wolhuter KM and Garner D. Interchange spacing in Gauteng, 3rd International Symposium on Highway Geometric Design. Transportation Research Board, Chicago, 2005.

Note: The configurations ‘Access’ and ‘Systems’ are defined in Chapter 11. The distances in the table are ‘white line distances’, which are discussed also in Chapter 11.

Links to freeways are often provided by minor arterials. They are invariably dual carriageways and connect to the local road system via traffic signal-controlled at-grade intersections. To achieve ‘green wave’ operating conditions on minor arterials their intersections should be spaced at roughly 500 metres, centre-line to centre-line but can actually vary from as little as 200 metres to about 1000 metres in the central business district up to as much as 5000 metres in the outlying urban areas. As a general rule, all arterials not classified as major arterials are minor arterials.

Local streets include through streets, crescents, culs de sac and dead ends. Speeds are low and the principal function of local streets is to provide access to adjacent properties and a link from these properties to the higher order roads in the network. They constitute the lowest level of mobility and do not usually contain bus routes. They can fall prey to rat–running, and through movements are actively discouraged and, where possible, totally eliminated.

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