Road Classification System in Nigeria

Road classification is a technique for grouping roads according to their purpose and location etc.. This strategy aids in organizing and prioritizing road maintenance and construction, as well as improving the effectiveness and safety of transportation. Before a road is classified, it is necessary to consider the factors that affect road classification systems.

Factors that Affect Road Classification Systems

There are about five (5) factors that influence the classification of roads in any system. These include:

i. Trip purpose: This refers to the reason why people use the road either for access or mobility.

ii. Trip length: This refers to the length of the road.

iii. Size and type of population centres served: This refers to the population served which could be cities or rural areas.

iv. Traffic characteristics: This refers to the nature and volume of traffic and also guide road classification.

v. Network and system requirement

Road Classification in Nigeria

Roads in Nigeria are generally classified based on two major criteria. These are:

A. By Ownership: This refers to the organization that owns the road because no individual owns a road. Under this, we have:

a. Federal highways

b. State roads

c. Local roads

d. Rural roads

B. By Function: Road performs two major functions. These are mobility and access. By mobility, road enables people and goods to move from place to place. By access, road enables people to gain access to strategic locations.

The functional classification of roads in Nigeria is as follows:

i. Class A: National Trunk Roads: These are roads that link provisional capitals, main centres of populations, and nationally important centres. The major function is to provide mobility. They require longer trip lengths and high design speed.

ii. Class B: Primary Roads: These are roads linking provincially important centres to each other or to a higher class road (urban/rural centres). Linkage between districts local centres of population and development areas with higher-class roads. The function is to provide both mobility and access. Class B roads require shorter trip lengths and intermediate speeds.

iii. Class C: Secondary Roads: These are roads linking locally important centres to each other, to a more important centre, or to a higher class road (rural/market centres) and linkage between locally important traffic generators and their rural hinterland function is to provide mobility and access.

iv. Class D: Minor Roads: These are roads linking to minor cities (market/local centres) and all other trafficable roads. The major function is to provide access to land adjacent to the secondary road system. Class D requires short trip lengths and low-speed.

Control of Access

Access refers to the ability of a road user to access his/her destination through roads. As mentioned earlier, access is one of the two major functions of roads. A road user who wishes to travel from home town to a major city would require various kinds of access roads starting from class D probably to class A or B. These access roads link each other at junctions so control of access is very important in each class of road for some purposes.

High access to class A roads would result in major accidents, reduce the capacity of the road, and may cause the road to fail early. Control of access is required for class A roads because their sole functionality is mobility. Even in other classes, it may be required to a certain degree.

Table 1: Access control by function class of roads

C. Classification of roads based on design criteria (AASHTO system)

Based on AASHTO design criteria, roads can be classified as:

1. Freeways

2. Arterial roads other than freeways and

3. Collector roads

Minimum Design Speed of Design Designation of Some Roads

Design speed is one of the important criteria used for the geometrical design of roads. Design speeds are very important for the safety of road users and it is in some way dependent on the access of the road. Roads with high access require low design and vehicle operating speed to avoid accidents while roads with limited access may be designed with high speed to save cost and time.

Note: design speed and design traffic govern the functional classification of roads.

Class A: National Roads

• Class A1 (freeways in rural areas) – 100 (kilometre per hour) (kph)/ Average Daily Traffic (ADT) -15000 veh/day

• Class A2 (freeways in metropolitan areas) – 100 kph/ADT – 20000 veh/day

Class B: Primary Roads

• Class B1 (primary rural arterials) – 80 kph/ADT – (8000 – 10000) veh/day

• Class B2 (primary metropolitan) – should be designed in the context in which it operates

Class C: Secondary Roads: Two-lane single-carriageway roads

• Class C1 – 80 kph/ADT ˃ 4000 veh/day

• Class C2 – 80 kph/ADT – (500 – 4000) veh/day

• Class C3 – 60 kph/ADT < 500 veh/day

Class D: Minor Roads

• Minimum design speed = 40 kph.

Table 2: Typical design speed based on design classification (below)

Design Vehicle

In Nigeria, buses and heavy vehicles are used as design vehicles for cross-section elements while the car is the design vehicle for horizontal and vertical alignment.

Table 3: Dimensions of design vehicles (below)

Table 4: Turning radius of vehicles (below)

Table 5: Classes of vehicles used in traffic count (below)