Reducing the number of lanes can be a good idea

Andrea James writes about something that I have noticed and that is that reducing the number of lanes on a road, what is called a ‘road diet’, and replacing the lost ones with a center lane meant only for turning left, can produce many benefits, not the least of which is that it reduces the number of opportunities for accidents.


Road diets reduce what engineers call “conflict points,” or places where vehicles are most likely to hit something. On a four-lane road, left turns have six: getting rear-ended while making a right or left turn, sideswiping a vehicle while changing lanes to avoid a right or left turn, and crossing two lanes of oncoming traffic. A road diet that incorporates a left turn lane cuts conflict points in half.

In Cleveland, Carnegie Avenue is a major artery that connects University Circle to downtown. It used to have three lanes going each way. I hated driving on it because the lanes were narrow and cars would be constantly changing lanes because of parked vehicles and the many intersections where left turns were possible and that had stopped traffic.

A few years ago, traffic engineers reduced the number of lanes to two each way with a center lane for turning left only. The reduction in the number of lanes from six to five enabled them to make the lanes a little wider. I now find it much pleasanter to drive on.

Other benefits are that with fewer lanes traffic moves more slowly and one has the option of creating bike lanes with the extra space. In my suburb, there were some residential streets where there was only a single marked lane going each way but the lanes were wide and even though the posted speed limit was 25 mph, you would have impatient drivers using the wide lanes to pass other drivers and thus enable them to go faster than was safe. Recently, traffic engineers have marked out a specific bike lane that takes up some of the width and this has resulted in no passing and much pleasanter driving.


  1. Sunday Afternoon says

    Mano – your post has reminded me of the (discontinued?) practice in the UK where some country roads had 3 lanes. I recall roads in the south of England in the 1980’s (specifically in Dorset) while on holiday with my parents.

    It was predicated on drivers observing “lane discipline” (keeping left unless overtaking). The travel lanes were each drivers’ left lane with the centre lane being for overtaking if clear. In hindsight this was a recipe for head-on collisions, so I’m pretty sure this has been discontinued.

    More on-topic – the M8 motorway (freeway) through Glasgow in Scotland used to have a 70 mph speed limit all the way through the city, including over the Kingston Bridge. I found it a fun, if thrilling drive. Quite a number of years ago now the speed limit was reduced to 50 mph and enforced with speed cameras, etc. As a result, it is a much more pleasant drive (and presumably safer). I’m pretty sure I read that the throughput of traffic (vehicles/hour) has gone up overall even though average speeds have reduced. Presumably, this is due to the lower speed requiring less reaction and braking space around vehicles so the vehicles are more closely packed when traveling at the speed limit.

  2. says

    The four lane road has six conflict points and the three lane road has three. While that might result in less collisions in total, the collisions themselves will be deadlier.

    Notice that the four lanes don’t have head-on collisions as a conflict point while the three lanes do. The amount of force resulting from such collisions is much bigger than that of being rear-ended or being hit from the side. I’m not so sure that switching frequent but survivable for infrequent but deadly is a doug trade-off.

  3. Trickster Goddess says

    The seriousness of head-on collisions would be mitigated because cars in that lane would generally be slowing down to turn left.

  4. hyphenman says


    Years ago,particularly in Pennsylvania but I’m sure they had them elsewhere as well, three lane roads were common.

    The third, middle, lane, was known as the suicide lane because people, rather than using the middle lane for turning left (this was before we painted arrows everywhere) drivers used the lane as a passing lane and head-on collisions ensued.

    John Irving used a common a safety rule at the time—always keep your wheels pointed straight ahead when waiting to make a turn in the turn lane lest you get rear ended and pushed into oncoming traffic—as a plot point in his book A Widow For One Year.

    Jeff Hess
    Have Coffee Will Write

  5. says

    @Trickster Goddess

    I’m not so sure about that. The only way to know for sure would be to examine a highway that implements this idea, keep track of both the total number of accidents and the total number of fatalities and compare it to more traditionally built roads.

    All the (admittedly few) articles I’ve read about the road diet concept point exclusively to the reduction of collisions as proof of it’s success. None of them mention anything about the severity of the accidents that do happen, which is a terrible omission. Less accidents =/= less deaths. Head-on collisions account for 2% of crashes and 10% of driving fatalities. Having them as potential conflict points in a road design should be treated with more caution instead of assuming that people will just “slow down”.

  6. machintelligence says

    Mountain highways here in Colorado have been adding a third lane (because widening to four lanes in the mountains is prohibitively expensive.) Even these third lanes are not continuous but are installed where feasible. They are for passing while going uphill only and downhill traffic is forbidden to use them. It has markedly improved the flow of traffic on the high passes and has reduced collisions from cars trying to pass slow moving trucks. All four land mountain roads are divided highways (in Glenwood canyon on I 70, remarkably so — sometimes the separation is vertical as well as horizontal.)
    And of course there are the runaway truck ramps.

  7. MarkDF says

    Where I live, left turns lanes are pretty much standard. I assumed that they were the standard everywhere, until I met Americans who were unfamiliar with them and were terrified by the prospect of facing directly into oncoming traffic. But the oncoming traffic is always either facing a red light, or turning left — so there’s actually no danger at all.

  8. Crimson Clupeidae says

    Tucson has done exactly what is shown in the figure to many of its roads. Here, it was done mostly to add the bicycle lanes. I don’t know what effect it had on traffic flow, which is more important (as it is was roads are designed for) or the accident rate. I know I tend to avoid those roads even more now because all it takes is one driver going really slow (we have lots of snowbirds here) to really screw up traffic.

    It is nice for riding my bicycle, although I now live close enough to a MUP that I barely venture onto busy streets.

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