Design & Functional Classification of Roads and Highways

Design & Functional Classification of Roads and Highways

Road classification system groups roads a limited number of clearly defined types.

Purpose of Road Classification

A road network is composed of various types of roads, each of which performs a particular service in facilitating vehicular travel between points of trip origin and destination, and in providing access to property. Road classification is the orderly grouping of roads into systems according to the type and degree of service they provide to the public.

Consideration for classification - Factors affecting classification of roads

Many different classification systems have been introduced and used for particular purpose. The basis for some of these classifications is:

1. Legal control
2. Surface type
3. Function
4. Geometric elements
5. Location
6. Traffic volume
   
   
   
   Seldom do these classification system differentiate between roads on basis of service which is is essential for road designer.

Design classification system

1. This system separates roads;
   • On the basis of differences in traffic service and land services.
   • On the basis of geometric design features.
     
   The eight primary divisions in this system are:
   

   Rural	Urban
   Local	Local
   Collector	Collector
   Arterial	Arterial
   Freeway	Freeway

1. Rural and urban:

It refers to predominant characteristic of the adjacent land use and not only to jurisdictions boundaries or features of typical cross-section.

2. Geometric design:

For geometric design purposes it is essential to divide each of these divisions. For a given road geometric design elements are affected by traffic volume. However it is significant for classification only in determining the number of lanes and whether road should be divided or undivided.

3. Other variables

Affecting geometric design are;

1. The type of terrain road passes through
2. Financial resources available
3. Population density

Development characteristics of the land surrounding, the road and the travel habits of the local population.

4. Design speed is the measure of quality and is therefore the final sub division.
   
   
   

Road Classification

1. 
   

   	Design speedKm/h	Local	Collector	Arterial	Freeway
   RURAL	50::130	RLU*50
   		RLU60	RCU60
   				RAU80
   		RLU100	RCU100
   					RFD100
   				RAD130
   URBAN	30	ULU*30
   		ULU50	UCU50	UAU50
   			UCU80		UFD80
   					UFD100
   					UFD120

   
   
   * UAD100:- Urban, Arterial, Divided, 100km/h (design speed)
   Freeway is preferred because it is more descriptive of the type of the traffic movement.
   
   

Factors affecting highway classification

1. To identify the classification to which any road belongs, the following factors should be considered.
   
   1. SERVICE FUNTION:- Most roads provide service to traffic, access to land or both. Following road type provide the service function given as;

      Freeways and Arterials ==> provide the movement of through traffic.
      Local roads ==> are used almost exclusively for land access.
      Collectors ==> provide a combined service.

   2. TRAFFIC VOLUME:- The low and high traffic volumes are carried by different roads. However, the volume rage for each classification is wide and overlaps that of other classification.

      Freeway and arterials ==> carries high volume
      Local and Collectors ==> carries low volume

   3. FLOW CHARACTERSTICS:- The desire characteristics of traffic flow determine the classification of road, e.g.

      Freeways and Rural Arterials ==> serve primarily uninterrupted traffic flow characteristics.
      Locals Roads ==> provide full land service which is restricted by traffic crossings, entering and leaving road by parked vehicles and in urban areas by pedestrians.

   4. RUNNING SPEED:- in an ideal road system, local connect with collectors, collectors connect with arterials and arterials with freeways. It is preferable to minimize the interconnection of locals with arterials and collectors with freeways as it can cause inconvenienced to the drivers and may increase in accidents.