Lane Width and Cars Per Hour Per Lane

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I am not qualified to teach anyone about highway engineering, I have a trade-based background, and am only providing references to what you may find helpful in understanding the link between car lane width and number of cars per lane.

First from the U.S. Department of Transport – Lane Width: Flexibility in the AASHTO Guidelines. Urban areas show a high degree of flexibility. It is noted that lane widths “may vary from 10 to 12 feet (3.0 to 3.6 m) for arterials.”

For urban roads in New Zealand, kerbside lane widths on straight alignments must be 3.1 m minimum. All other lanes must be at least 3.0 m wide and on corners the lanes should be widened by between 0.3 to 0.9 m as the curve radius changes from minimum R60m to R300m. You can see more details in Table 6.5 in the link below.

The ideal lane width is stated as 3.5m, with 1.8m clearance to fixed obstacles close to the road. When these measurements are reduced, traffic flow is reduced, as shown in the table below from this Transit NZ draft Highway Manual – State-highway-geometric-design-manual Section 6.2 Traffic Lanes

Clearance to fixed obstacle close to road Lane Capacity (% of 3.5m lane capacity)
3.5m lane 3.3m lane 3.0m lane 2.7m lane
1.8 100 93 84 70
1.2 92 85 77 65
0.6 81 75 68 57
0.0 70 65 58 49


The following extract from the Canadian journal of Transportation
Vol 4, No 1 (2010) gives some idea of nominal flow.

Abbreviations: Highway Capacity Manual (HCM) and Passage cars per hour per lane (pcphpl)

For lane width, the HCM recommends 3.66m in the ideal case and a reduction of capacity for reduced width. Agent and Crabtree (1983) indicated that lane width might not have any effect on saturation flow for widths more than 3.0m and suggested a 5 per cent reduction in saturation flow for lane widths between 2.7 and 3.0 m. For commercial vehicles no effect was observed even for lane widths below 3.0m. Zegeer (1986) evaluated saturation flow rates on approaches with lane widths varying between 2.6 and 4.7m, and found that saturation flow rate reduces by 2 to 5 per cent for narrower lanes and increases by 5 per cent for wider lanes as compared to standard ones. Potts et al. (2007) investigated the relationship between lane width and saturation flow rate on urban and suburban signalized intersections and observed that saturation flow rate varied significantly with lane width. Average saturation flow rate ranged between 1,736 to 1,752 pcphpl for 2.9m lanes, 1,815 to 1,830 pcphl for 3.3 to 3.6m lanes, and 1,898 to 1,913 pcphpl for lane widths of 4.0m or greater. These saturation flow rates are generally lower than those currently recommended by the HCM. Lewis and Benekohal (2007) suggested that adjustment factors vary from 0.82 to 1.13 for lane widths ranging between 2.8 and 4.0 meters.

The question to ask now is what is the effect on road safety?

My first reference on this comes from the US highway manual Chapter 3.

In a reduced-speed urban environment, the effects of reduced lane width are different.  On such facilities, the risk of lane-departure crashes is less. The design objective is often how to best distribute limited cross-sectional width to maximize safety for a wide variety of roadway users

And here is a report on Relationship of Lane width to Safety for Urban and Suburban Arterials, presenting crash data from 590 crashes in Michigan, on two-lane, undivided arterials (2U). There was no change in fatal and injury crashes between 10 foot-wide (3.0m) and 12 foot-wide car lanes (3.3m).

Lastly, the following image from the Greater Wellington Council on survivable-speeds suggests there is little risk of serious injury in car-on-car head-on crashes when speeds drop below 50 kmh.


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2 comments on “Lane Width and Cars Per Hour Per Lane

  1. Narrower roads are not less safe. Many studies have found that, with the exception of a few specific road layouts, drivers self-regulate when lanes are narrow, whereas complacency can happen when roads are wider. Beyond a certain width, single lanes are sometimes used as a defacto two lanes. This happens on Jervois Road in Auckland, near the intersection of Ponsonby Road and College Hill. This article outlines some other research and reasons why drivers have to be more cautious on narrower roads. Of course, most of the research is in the USA whereas New Zealand does have a lot of low quality rural unsealed roads.

  2. Science aside, Im not disputing statistics. From a road cycling perspective my experience is that the safest roads are those without cycle lane markings (so that it is not clear who ‘owns’ the space) that are one lane each way, wide enough not to accommodate two defacto lanes of traffic but for a truck to pass a cyclist within that lane with enough room to spare so that the ‘safety zone’ of the cyclist is not encroached, with the cyclist keeping left

    Felltimber Creek Road in Wodonga, Victoria is a good example. Yet in the same small city (I don’t live there now) there are also some examples of poor road engineering, Drage Road near the Victory Luteran School being one of several examples. This is where the shoulder of a road suddenly ends, forcing cyclists into the lane, right near a mid-road island such that cars attempt to pass cyclists dandgerously close to the ‘cyclist killing zone’.

    Narrow roads are only safe for cyclists where there are low speeds, which are typically hilly and windy roads, with short distance visbility, until someone tries to drive at speeds which does not allow reaction time for ‘surprises’. The road to Raglan from Hamilton is a classic example, a provincial highway, with no shoulders in many places, twisty and windy. Would make a fabulous cycling route from city to rolling sea/surf town but there is no space to ride safely in many places, such that cyclists can end up being a nuisance to other traffic, and as a cyclist, I am aware I have to share the space with others and never wish to be a nuisance, such that I avoid such roads. But they could be engineered better to be more inclusive.

    Cycling is part of my lifestyle, it keeps me fit mentally and physically, such that I am not a drain on the health system. It is as much a part of my living amenity as my expectation to use a car to go to work, or to have decent public transport options in those places it is feasible to provide them. Its pleasing to see in NZ that we seem to be entering a more enlightened engineering era that now seems to try to accomodate, rather than deliberately exclude cyclists, as occurred in the past. Its occurring more off-road than on, but in Hamilton strides have been made to try to accommodate cycling which is good to see. Its means a healthier and nore inclusive city. Its more recreation and sport/health options, and a bettr living amenity. Its good for business too, to have large numbers of cycling. Especially if you own a cafe, which I don’t but frequent often after a decent ride.

    Now we need a concentrated driver training regime and cycling courtesy program to occur to facilitate this.

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