What Is Lane Width?

Definition

The lane width is the width of one traffic lane on a road. It is a road-design value. Designers choose it together with the shoulder width and the design speed.

How highway design sets it

The AASHTO Green Book gives a range of 9 to 12 ft (2.7 to 3.7 m) for lane width. The designer picks a value from the desired speed, capacity and context. Roads have used 12 ft lanes for a long time. The Green Book allows 10 ft lanes where speeds are 45 mph (about 72 km/h) or less. FHWA reports both points from the 2011 edition.

Designers once avoided narrower lanes because of concerns about safety and congestion. FHWA reports research on suburban and urban arterials that found no negative effect on overall safety or operations from 10 ft and 11 ft lanes. These lanes had no measurable effect on vehicle capacity. The study found one exception. Designers should use 10 ft lanes with caution on 4-lane undivided arterials.

Why lane width matters for safety

Wide lanes help in two ways. They increase the separation between vehicles in adjacent lanes. They also give a driver more room to correct in a near crash. FHWA quotes ITE that the risk of head-on and sideswipe crashes across the centerline increases as the lane gets narrower.

The effect depends on the road. FHWA reports that lane width is a safety factor on rural two-lane and rural multilane roads. It reports no substantive safety benefit between 10, 11 and 12 ft lanes on urban arterials with posted speeds of 45 mph or less. Narrower lanes can also lower the operating speed on an urban street.

Lane width outside the US

Other countries set the value in their own road standards. The Spanish standard Norma 3.1-IC says the usual lane width is 3.50 m. A designer can reduce it, with a reason, on peri-urban and urban sections, together with a lower speed. On a road with separate carriageways, the left lane can be narrower than the right lane because heavy vehicles use the right lane more often. Table 7.1 of the standard gives 3.50 m for most road classes and speeds. It gives 3.00 to 3.50 m for a conventional road with a design speed of 50 or 40 km/h.

Design exceptions

On the US National Highway System, lane width is one of 10 controlling criteria. Only design speed and design loading structural capacity apply to all NHS roads. The other eight apply only to high-speed roads. These are Interstates, other freeways and roads with a design speed of 50 mph (80 km/h) or more. A project that does not meet a controlling criterion needs a design exception.

On an airfield

Runway width is a separate value that aerodrome rules set. It is not a lane width, and the road values do not apply.

Runway width in the EU and ICAO rules

In the EU, CS ADR-DSN.B.045 gives the minimum runway width from the code number and the outer main gear wheel span (OMGWS). The OMGWS is the distance between the outside edges of the main gear wheels. ICAO Annex 14, section 3.1.10, gives the same table as a Recommendation. The minimum widths are:

• Code number 1: 18 m where the OMGWS is below 6 m, and 23 m where it is 6 m up to 9 m.
• Code number 2: 23 m where the OMGWS is below 6 m, and 30 m where it is 6 m up to 9 m.
• Code number 3: 30 m where the OMGWS is below 9 m, and 45 m where it is 9 m up to 15 m.
• Code number 4: 45 m where the OMGWS is 6 m up to 15 m.

A precision approach runway of code number 1 or 2 is at least 30 m wide. The code number comes from the aeroplane reference field length. The code letter comes from the wingspan. ICAO Annex 14, Table 1-1, gives both.

EASA measures the width at the outside edge of the runway side stripe marking where one is provided. Otherwise it measures at the edge of the runway. A side stripe is at least 0.9 m wide on a runway 30 m or more in width. It is at least 0.45 m wide on a narrower runway.

Runway width in the FAA rules

FAA sets runway width by runway design code (RDC) in AC 150/5300-13B, Appendix G. For approach categories A and B, the width is 60 ft (18 m) for small aircraft of group I. It is 75 ft where the visibility minimums are lower than 3/4 mile. It is 100 ft for group III and 150 ft (46 m) for group IV. See runway width .

Shoulders and taxiways

CS ADR-DSN.B.125 says to provide runway shoulders for code letter D, E or F. This applies where the OMGWS is from 9 m up to 15 m. Under CS ADR-DSN.B.135, the overall width of runway and shoulders is not less than 60 m for code letters D and E. For code letter F, it is 60 m with two or three engines and 75 m with four engines.

Taxiway width follows the same method. For a straight taxiway, CS ADR-DSN.D.245 gives 7.5 m, 10.5 m, 15 m and 23 m for the four OMGWS bands. EASA measures it at the edge of the paved surface. Where the taxiway edge is marked, it measures at the outside edge of the taxiway edge marking.

Markings and lights that depend on the width

The runway width sets the number of stripes in the threshold marking. CS ADR-DSN.L.535 gives 4 stripes for 18 m, 6 for 23 m, 8 for 30 m, 12 for 45 m and 16 for 60 m. The stripes start 6 m from the threshold. They extend sideways to within 3 m of the runway edge, or to 27 m on either side of the centre line, whichever is smaller.

Runway edge lights are in two parallel rows, equidistant from the centre line. CS ADR-DSN.M.675 places them along the edges of the area declared as runway. It allows them outside the edges by not more than 3 m. On an instrument runway the spacing in a row is not more than 60 m. Where the area that could be declared as runway is wider than 60 m, the row distance depends on the operation and the light distribution.

Paving lanes

“Lane” does appear in airfield paving. A paving lane is the strip that one paver pass places. FAA says paving lane widths and the position of in-pavement light fixtures affect the joint layout of a concrete pavement. For asphalt, AC 150/5370-10H, Item P-401, requires the mix to go down in consecutive adjacent lanes. The project specification sets the minimum lane width.