5.4 Carriageway widths and tracking (swept path analysis)

Wide carriageways encourage faster speeds and consume large amounts of land and resources.

We must create carriageways no wider than is absolutely essential for vehicles to pass and ensure access for fire appliance.

Factors that affect the width of a carriageway include volume of vehicular traffic, informal on-street parking, speed limit, demarcation with pavement and the street’s curvature.

Designers should be encouraged to vary carriageway widths, particularly where a rural character is desired. This allows for less formal opportunity parking and allows the street layout to respond to the nature of the built form.

Figure 5-4: The built form should determine the carriageway design (Credit – Manual for Streets)

It is important to consider the street beyond the carriageway edge, and width not solely as a function of vehicle space and parking. A street is a linear space defined by the buildings which enclose it.

The depth of plot frontage (i.e. front gardens) and building height therefore needs to be taken into consideration when determining overall widths. It is recommended that most streets should have an enclosure ratio (building height: width between buildings) of between 1 : 1 and 1 : 3. This will provide a good sense of enclosure that people will find comfortable and pleasant. Streets wider than this may feel like a racetrack, encouraging higher vehicle speeds. Those that are narrower may feel claustrophobic.

Figure 5-5: Street sections with enclosure ratios of 1 : 1.5, 1 : 3 and 1 : 1 (Credit – NMDC)

Lightly trafficked two-way streets, i.e. secondary streets (without buses) and lower, should have a carriageway width of 4.1m – 5.5m, excluding any on street parking bays.

In secondary, local and tertiary streets it is acceptable to have larger vehicles taking up more than one lane, providing cyclists can still pass safely and there are spaces for vehicles to pull in to allow oncoming traffic to pass.

Recommended widths, parking arrangements, materials, etc. are set out in the Street Typology table (4.14).