How are road widths decided?
Road widths are decided using:
- Vehicle type
- Volume (frequency)
- Speed
- Lane type required (e.g. is a turning bay required, or will long vehicles need to turn sharply and therefore need a larger swept path)
- Situation adjacent to the lane (e.g. parking, kerbs, verges, street furniture)
- Cross fall (slope of the road)
- Horizontal alignment (i.e. curves)
- Provision for other modes (e.g. pedestrians, cycles, buses)
A typical car is 1.9m wide plus wing mirrors and the maximum width of a vehicle is 2.5m wide plus mirrors (unless it has an over-dimension permit). Roads are constructed so that each lane is wide enough to accommodate standard sized vehicles plus a margin of error as it’s difficult to keep the swept path of a vehicle completely within its width.
The theoretical operation of heavy vehicles within a traffic lane has been modeled on a computer. The majority of heavy vehicle configurations require less than 3.2m lane width at 90kph and 3.1m at 60kph.
What are standard road widths in Australia?
Widths can vary from territory to territory, so the following are standard guidelines. A standard urban lane is 3.5m wide, but there are many lanes which have reduced widths for various reasons, e.g. deliberate narrowings to encourage drivers to reduce speed, or constraints where room has been allocated to cycle lanes. There’s no evidence in Australia that slightly narrower lanes reduce speed.
Despite having an enormous country, our road widths are slightly less than many other countries. Our general traffic lanes are 3.3-3.5 metres for roads with higher speeds and truck volumes, or 3.0-3.3 metres for lower speed roads with low truck volumes. In comparison, countries such as Brazil, China, Germany, Hungary, Japan, Switzerland and the UK have between 3.5-3.75 on freeways and 3.25-3.75m.
Are wider lanes safer?
There’s no conclusive evidence that wider lanes are safer. When lanes become too wide, they become less safe. Crash frequencies increase once lane width exceeds 3.4m, and have statistically significant higher levels of accidents at 3.7m and above.
One possible reason for this is that drivers might increase speed with wider lanes.
Parsons Transportation Group (2003) found “lane width does not appear to be correlated to collision rate. Narrower lanes have been both credited for reductions in collisions and blamed for increases in collisions. In both cases, lane width alone is not the primary cause of changes in collision rate.”
However, TMR Queensland (2010) research found that there was a reduction. Nilsson (2001) found that there is a speed difference of around 0.4kph per extra metre of traffic lane width, therefore it’s negligible.
Do wider lanes mean faster traffic flow?
A study by Petritsch found that traffic flow is determined by junctions, not lane width: “so long as all other geometric and traffic signalization conditions remain constant, there is no measurable decrease in urban street capacity when through lane widths are narrowed from 12 feet [3.7m] to 10 feet [3.0m]”
Road damage
Narrower lanes wear more quickly because the tyre tracks of vehicles are constrained into a more narrowly defined space. Degradation happens 20-40% more quickly (respectively) when a road is reduced from 3.66m to 3.35m or 3.05m according to Liu and Wang (2003).
Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists
