This is one of a series of posts on highway engineering. It discusses roads in terms of hierarchies. The text comes from a google knol published in January 2009. The other material from this google knol will be added as separate posts.
Volkswagen and Honda basically sell the same models of their cars all over the world. If the same vehicles travel the roads in different countries then theoretically the roads should be the same. However there is the impression that roads differ between countries. This is partly due to the impression that design standards vary from country to country, and also because there is no common form of road classification. Indeed road classification, and the terminology used to describe the classes, is not clearly defined or permanent even within one country.
An important publication on road hierarchy, the UK government’s 1963 report on ‘Traffic in Towns’ (‘the Buchanan Report’) discussed this question of names for different types or classes of road. It suggested there should be a simple set of names which could replace “the present large number of terms – arterial roads, through roads, expressways, freeways, principal traffic roads, collector roads, service roads etc. – which are freely used with little if any standardisation of meaning” an. Here Buchanan was referring to the terms used in one country.
The design/structure of road classification systems has arguably not kept up to date with the change in uses and types of roads. For example a road can be an urban single carriageway road, but at the same time
a cycle route
- a bus route
- a shopping street
- a main road for emergency services
It would be useful to develop a classification which would be applicable to each country and which could be computerised for use in (e.g.) a municipal urban GIS. The task should not be impossible given that there exist classifications for complex groups such as goods sold in supermarkets (the product “bar code”) and for the plant world. This document takes a brief look at some existing road classification systems around the world and suggests some ideas for the development of a standard international classification system.
A classification is a system for telling the reader something about the character and use of a road. A classification system is the ‘systematic placement in categories’, where categories are ‘a class or group of things, people etc. possessing some quality or qualities in common’ (Collins’ thesaurus). It does not necessarily arrange things in order, and so is not a hierarchy (which is defined as ‘a system of persons or things arranged in a graded order’).
A system of road classification can impart information by (e.g.) giving each class a code, as in the earlier UK system of class A, B and C roads; or by giving each class a descriptive title or term, such as
- rural freeway in mountainous areas
- suburban local roads
- high-speed rural roads
Engineers have been classifying roads throughout history, whether by using such general terms as ‘highways and byways’, or by specific terms such as the ‘Royal Road’ used by Persian King Darius the First, Weimar Germany’s ‘Autobahns’, America’s ‘Interstate Highways’, or today’s UK ‘motorways’. The terms were short forms for describing some of the engineering or functional characteristics of the roads.
- the Royal Road – was restricted to Royal messengers in order to prevent them being impeded by common travellers
- the US Interstate Highways – were inititally justified to US Congress after World War II as a national defence system, and this justification brought with it the requirement that its geometry and structure should be able to accomodate and aid the movement of large pieces of military equipment.
- motorways – a term which conveys the information that the road is designed to take long-distance traffic at high speeds along routes where the traffic flow is not disrupted by at-grade junctions and accesses.
Current practice with road classification
The following brief notes describe current practice in different countries.
A search through UK reference works, textbooks and design guides reveals a range of terms meant to indicate the different road classes. They generally fall into one of two groups: a code system where e.g. D4M stands for “dual 4-lane rural motorway”; and a descriptive system, with terms such as “all-purpose trunk roads”. There are however cases of what seem to be a mixture of the two types, e.g. “single 2-lane roads (categories 1 to 4).
India has the following road classification:
- National Highways (NH): Roads which connect cities between different states within the country. Earlier they were 2-lane undivided carriageways but now most of them are completed/ongoing to be widened to 4-lane divided carriageway and even some of these are even being widened to 6-lane divided carriageways.
- State Highway (SH): Roads which connect major cities within states. Mainly 2-lane undivided carriageways but some of these roads are being widened to 4-lane divided carriageways.
- Major District Road (MDR): Roads connecting major town/cities within a districts. Mainly 2 lane or some of these are intermediate lane (5.5m wide) also. Now these roads are being widen to 2-lane roads.
- Other District Road (ODR): Connecting towns within district, intermediate or single lane (3.75m) wide. Now these roads are being widened to 2-lane roads.
- Village Roads (VR): Road roads connecting villages within a district. Single lane roads which are being widened to 2-lane roads.
The website of Italy’s Ministry of Transport reproduces legislative decree (DL) no. 285 of the 30/04/1992. Article 2 of this decree describes the definition and classification of roads as follows:
A road is defined as a facility for public use and for the circulation of pedestrians, vehicles and animals. Roads are classified in terms of their constructional, technical and functional characteristics into the following types:
B: main inter-urban roads
C: secondary inter-urban roads
D: urban freeways
E: urban district roads
F: local roads
F2: pedestrian/cycle routes (this class added in 2003)
Road classes B,C and F above are further sub-divided into
A state roads
B regional roads
C provincial roads
D municipal roads (strade comunali)
As an example, DL 285 describes “A: motorways” as:
Inter-urban or urban roads with independent carriageways or carriageways which are separated by an insurmountable traffic barrier. Each carriageway (of a motorway) has at least two traffic lanes, and may have a paved hard strip to the left and an emergency lane or paved hard strip to the right. There are no at-grade intersections or private accesses. A motorway has fencing and emergency assistance facilities for users along its full length, is reserved for the circulation of certain types of motor vehicles and has appropriate traffic signs from over its full length. It must have suitable service areas and parking areas, both with entries/exits which have deceleration/acceleration lanes.
Generally there is no official functional road classification in Lebanon, except for what are called “autostrade”. This term describes is a multi-lane divided highway with full control of access. It is defined as such by law, but full control of access was never implemented in practice.
The Netherlands has a simple and direct way of classifying roads which is called “sustainable safety”. It differentiates between roads within the built environment and outside (e.g. city limits) and introduces 3 sub-classes or road types for each:
1. Flow roads (within limits: 70, 50 km/h; outside 100, 120 km/h)
2. Area Access Roads (within: 50 km/h; outside 80 km/h)
3. Home Access Roads (within: 30 km/h; outside 60 km/h)
There are specific recommendations or norms for design for each road type (e.g. bicycles on separate infrastructure or mixing of traffic types, width etc.). Designers have much more freedom designing class 3 roads (lay-out, materials, traffic calming measures) than class 1 and 2 roads. This type of classification had been discussed for some years in the Netherlands before it was officially implimented in 1997. Since then only small changes have been made (e.g. by distinguishing 2 types of class 3 roads, where one has more capacity and one is more for pedestrians, playing children and cyclists)
Notes by knol author:
- The speeds listed above almost cover the full range of speeds (or speed limits) seen on European roads. These increase in steps of 10 km/h from ~ 0 to 130 km/h. (The argument that design recommendations can be closely specified for each road type/speed limit is discussed in the knol on “design speed“)
- In terms of highway geometrics it can be argued that there is no difference between urban and rural roads (see the knol on “urban and rural roads“)
- Perhaps “flow roads outside city limits” could also include for design speeds well below 100 km/hr, in the sense that in extensive rural areas with low population high-speed roads could be disproportionately expensive
Roads in Slovenia fall within 2 main groups. State Roads are the responsibility of the central government, which funds, designs, builds and maintains them. Local Roads are the responsibility of the municipalities, which also fund, design, build and maintain them. There are also some some 10 defined road classes. However despite the term ‘class’ in their names (see table), they represent more a level in Slovenia’s road hierarchy than in a classification system. An exception perhaps is the two ‘groups’ which explain the roads administrative and financial background.
| state roads
|| local roads
| A – motorways
HC – expressways
| G1 – major roads I
G2 – major roads II
| R1 – regional class I
R2 – regional class II
R3 – regional class III
| LC – local roads
JP – public paths
Table 4. Road classification, Slovenia
Germany has a system of codes in which for example E(V) – is a local access road inside a built-up area with adjacent residential buildings.
There is also another system of codes, in this case representing a series of standard cross-sections. Each is directly linked to design speeds, ranges of traffic flow and geometrical features. For example RQ 35.5 = a dual 6-lane carriageway with verges and hard shoulders, and with an overall cross-sectional width of 35.5 metres.
In the US roadways are classified in general by Municipal Planning Organizations (MPO). Here the reference is to the functional classification of roadways. There are three basic categories that are again subdivided: 1. Arterial – Primary routes for goods movement from one city to another, 2. Collector – roadways that funnel local traffic to the arterial system, and 3. Local – local low volume street system. For more details see the ewgateway.org page on functional classification.
Roads are also classified in terms of their relation to the public administration. Practice varies between countries, but the features which can be covered by an administrative classification of a road include:
- who owns the road (state, region, district)
- who is responsible for designing the new road
- who is responsible for maintaining the existing road
- who pays for the construction
- who pays for the maintenance
Suggestions for a road classification system
A system of road classification should tell people something about a road’s physical characteristics, cross-section and its use. It should therefore cover some or all of the following characteristics of the road:
- types of user
- number of traffic lanes
- direction (one-way or two-way roads)
- number of carriageways (single, dual)
These terms may repeat the sense of others: for example, the function of motorways is to allow traffic to travel at high speeds (so ‘speed’ would be something of a duplicated characteristic); and the number of traffic lanes is an indication of the road’s capacity. One possible road classification system, based on codes, could be reasonably based on as few as 4 characteristics. The first field mainly affects the road’s position in the network road hierarchy, and its engineering design values (such as lane width); the other three mainly affect its cross-section:
- number of carriageways (single, dual)
- number of traffic lanes (which is related to the road’s capacity)
- design speed (which is related to the road’s function, its engineering geometrics and the speed limit), and
- direction (one-way or two-way roads)
Road classification – short descriptions code
Perhaps a road classification system could be developed based on a “short descriptions” code. For example the UK has an incomplete system which refers (e.g.) to a dual-carriageway motorway as a ‘D2M’, but this doesn’t indicate the design speed. An alternative code would include design speed, as shown in the following table.
|dual/single carriageway||number of traffic lanes in each carriageway||design speed||other features||example|
Table 5: Elements of a short descriptions code
The roads within an area or country constitute a physical road network. They also represent several different types of network – for example vehicle type networks (cycle networks, goods vehicle networks). For the same area, the roads in these “type” networks will vary from type to type. Further, the roads in one area will represent several different type networks
Road networks are often referred in terms of a road hierarchy. Often a hierarchy groups roads in an area into sub-classes based on their function, with (for example) roads intended for longer-distance traffic being the upper class, and access roads the lower class. This can give decision-makers the mistaken impression that (for example) upper class roads are the most important. Use of road hierarchies should therefore be avoided.
References and sources
1. Details for the Netherlands were suggested by Marco Van Burgsteden.
2. Details taken from the website of Italy’s Ministry of Transport and translated by the knol author. Thanks to Marco de Mitri (www.marcodemitri.it) for the suggestions.
3. Notes on India kindly provided by Engineer Deepak Dutt Sharma