## Width of a traffic lane (3) from first principles

One of the ways to estimate traffic lane width is to work from first principles. In a previous blog post about cycle lane widths (here) I said that the width of a cycle lane could be calculated from ….

• a) the shy distance from the edge of the road
• b) the width of the vehicle
• c) the inter-vehicle distance between bicycle and vehicles in the adjoining traffic lane

This is a very basic way to estimate lane width. We could apply the same basic way to lane width for motor vehicles.

Shy distance from the edge of the road

Taken as zero, although there are indications that obstacles close to the edge of the road affect speed, and designers are sometimes ad vised to add clear space between edge of lane and such obstacles, to reduce this effect (see e.g. here). Also, Austroads (ref. 1887) says that:

“Road authorities may also choose to provide an additional clearance to the face of the kerb to account for shy line effects, or for kerb channels that have a wider channel (e.g. 450 mm) in areas of high rainfall).

Width of the vehicle

Maximum truck widths in Europe are 2.55m (see ref. 1726), although refridgerated trailers can be 2.60m wide. These figures do not include the width of wing mirrors (see e.g. here). Figure 235 in (ref. 1617) indicates wing mirror additional widths of 2*0.25m, so that the width of a refridgerated truck would be 3.10 m.

Inter-vehicle spacing

Figure 235 in (ref. 1617) shows this as 0.25m for most combinations of motor vehicles, although for passing buses the figure shown is 0.40m.

Traffic lane width

This would put the traffic lane width as

• a) = 0
• b) = 3,10 m
• c) = 0,25 m

So that traffic lane width := 3.35 m.

Comment

I have probably over-simplified the calculation. For example, additional lane width would be needed to cover

• Vehicle wobble – vehicles do not travel in a precise and consistent line, the path they take wobbles slightly. (Ref. 1916) refers to this as “normal driving wander”
• Vehicle tilt – for example on motorway ramps, trucks with high loads when taking the ramp curve can tilt, and this would add to the effective vehicle width
• Curves – large vehicles take up more road width when travelling along a curve
• Changes in inter-vehicle spacing – I would think that, at higher speeds, drivers would feel safer if there were more inter-vehicle spacing, so that a lane width of 3.50 m at 100 km/hr might have to be widened to (say) 4.00 m at 130 km/hr to maintain a similar degree of safety. However this does not seem to be the case. (Ref. 1887) says for example that

“The provision of standard lane widths of 3.5 m allows for large vehicles to pass or overtake, without either vehicle having to move sideways towards the outer edge of the lane. Research has shown that there is no evidence that supports the assumption that road safety is increased with wider traffic lanes”.

…. and Germany has motorway standard cross-sections which show 3.50m lanes where speeds might be in excess of 130 km/hr (Ref. 1615)

Ref. 1617 has some quite detailed notes on this type of approach in its section 4.4.5 “Nutzungsansprüche an Straßenräume” (~ usage requirements and road width).

References

1615 – Germany, “RAA Richtlinien für die Anlage von Autobahnen”, fgsv 2008

1617 – Germany, Wolf et al, “Strassenplanung”, Werner Verlag, 2013

1726 – International Transport Forum “Permissible maximum dimensions of trucks in Europe”, 2013

1887 – Australia and New Zealand, “AGRD part 3: Geometric design”, Austroads, 2010

1916 – PIARC, “Cross Section Geometry in Uni-directionnal Tunnels”, 2001