Roadway Structures: Types, Definitions, Meanings & Uses | Travelledway, Auxiliary Lanes, Shoulder, Medians, Border Area, Verge

Roadway structure is the arrangement of the various cross-sectional elements of the road. It is therefore pertinent to know what these cross-sectional elements are before discussing roadway structures.

The following are the elements of a highway cross-section:

The travelled ways (American) or Roadway (British), Carriage ways (British) auxiliary lanes, shoulder, medians (also called central reserves or central verges), verges (British only) and border areas (American only). 

The width of, and member of the various elements vary with the type of highway. It is these that make the difference between one roadway structure and another.

Let us now look at the definitions/meanings of these roadway structures:

1. Travelledway or Roadway: This is the portion of the roadway (carriageway) available for vehicular traffic movement. It is normally composed of two or more traffic lanes except on ramps where one lane may be sufficient for traffic movements. On highways where the number of lanes is up to four and more (expressways and freeways or motorways), it is usual that opposing traffic streams are separated by the provision of a media or central verge to result in a dual facility or divided highway.

2. Auxiliary Lanes: An auxiliary lane is the portion of the roadway adjoining the carriageway/travelledway provided for parking, speed change, turning, storage for turning, weaving, truck climbing or for such other purposes than for through traffic movement. Usually an auxilliary lane measures 3.65m wide.

3. Shoulder: The shoulder, in American terminology has three meanings depending on its modifying objectives viz:

                 Graded, Surfaced and Usable shoulders.

The graded shoulder is the width or space between the outside edge of the roadway (carriageway) or auxilliary lane (if provided) and the intersection of the shoulder slope with the side slope plane.

The surfaced shoulder is that width outside the through traffic lane (or outside lane of carriageway). This is usually paved in most cases, in surface dressing.

The usable shoulder is the actual width usable for an emergency stop, including some of the rounding at the top of the flat earth slope. It is however classified as hard or soft shoulder depending on whether it is paved (hard) or just grassed or turfed. Soft shoulder width ranges from 1.8m (6ft) to 2.75m (9ft). On expressways, shoulders are very necessary. They are desirable on urban roads but cannot always be provided because of space constraints.

A well designed and adequately maintained should will serve the following functions:

a) Provision of space for drivers to recover safely if they lose control of their vehicles.

b) Provision of space for emergency stop off the travelledway, thereby reducing the risk of road accidents.

c) Making for ease of driving through a sense of openness created by the extra width.

d) Provision of space for vehicle maintenance operators as well as rest.

e) Enhancement of structural support of the through traffic pavement by the edge protection provided by the shoulder, especially when paved.

f) Improvement of highway capacity.

4. Medians (Central Reserves or Central Verges): They are portions of the roadway provided for the separation of the travelledways on high-volume, high speed highways (freeways or motorways and expressways) having four or more lanes. The median may also be referred to as MEDIAN STRIP in British English. Median width vary between 0.6m (2ft) and 30m (100ft) depending on the type of highway, the lower width being for urban areas while the wider widths are for expressways and freeways otherwise called motorways.

When independent roadway design is used for the separated carriageways, median can be of continually varying width. Median may be point-striped separations (0.6m t0 1.2m wide), narrow curbed section (1.2m to 1.8m wide) or swale section which is usually very wide.

In urban areas, the width between the raised curbs is usually paved if narrow, it is grassed and flowered, if it is wide enough to accommodate a maintenance roadworker without fear of being knocked down by a vehicle.

5. Border Area: It is the area between the roadway edge (or frontage road, if provided in an urban area) and the right-of-way line. It does not include the shoulders. its width should be such as to accommodate any necessary cut or fill slopes, ditches, walls, bikeways (or cycle tracks in British terminology) or sidewalks, and a landscaped buffer area between the highway and the abutting land uses. Along city streets, it might include an area for placement of utilities. The border area depends on the width of the right-of-way, which in turn, depends on the type and class of highway. Normal width ranges from 1.22m to 2.44m (plus sidewalk) in urban roads, 4.5m to 7.5m on rural roads other than expressways and freeways (or motorways), and 9.0m to 18m for rural expressways and freeways (or motorways).

6. Verge: According to Wikipedia, a road verge is a strip of grass or plants, and sometimes also trees, located between a roadway (carriageway) and a sidewalk (pavement). The verge may be taken as the British equivalent (but not quite) of the American border area because of its narrow width and the non-existence of the term “right-of-way” in British terminology. Its provision as level verge is recommended alongside all classified roads in rural areas. It should have a preferred minimum width of 3.65m of which 1.2m adjacent to the carriageway should be clear of footways, lighting standards and other obstacles which provide marginal friction and so reduce the capacity of the road.

Like border areas, verges may also incorporate footways and cycle-tracks. The former is only necessary in the viscinity of villages while the latter is necessary where average daily flow exceeds 1000 cycles per 16-hour day or where a heavy peak flow is anticipated, as might occur near schools and factories.