Optimum speed for moving cars

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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 ideal speed for a car lane.

Firstly, this is how Wikipedia describes traffic flow:

Due to the individual reactions of human drivers, vehicles do not interact simply following the laws of mechanics

Secondly, very useful information is available in NZTA Report 489-The costs of congestion reappraised-Feb 2013 This is most helpful in giving some idea of speeds to expect roads to be working at.

Note that the speeds shown here are not strictly comparable with the proposed approach which does distinguish between road types – the speed at capacity for urban streets is generally 30–35km/h while the speed at capacity for motorways is 75–80km/h

Earlier in the report a graph shows speeds below 50km/h when the number of cars per lane reaches its flow limit. You can find comments on this report at Transportblog 11Mar13


A third reference comes from a report “The role of speed–flow relationship in congestion pricing implementation with an application to Singapore,” which shows speeds also below 50km/h and some pretty efficient highway-type traffic flow.

Microsoft Word - Peter Bos, Blog Post - 2 off Hamilton medium Ci

If the ideal speed for optimum traffic flow is below 50 km/h in an urban environment, why do we have speed limits higher than this in a city?

We know that when we have 50 km/h human versus car conflict, the risk of very serious injury or death is very high. Not only that, urban roads have at-grade intersections, where we get cross traffic, which brings the risk of motor vehicle versus motor vehicle side impact crashes: the end result is increased motor vehicle occupant fatality.


The last word on side impact (T-bone) crashes comes from The Royal Auto Club of WA
RAC-Fact Sheet-Speeding

If there is a chance that cars may be involved in a side-on impact, the safe speed identified is 50 km/h.

2 comments on “Optimum speed for moving cars

  1. I’ve been developing a notion that cities should have a 30km/h baseline and have to provide justification for arterials to be higher rather than the other way around. We have too many fights trying to get speed reductions in town centres because safety is secondary to speed.

  2. Yes the 30 km/h has been seen as normal in most northern European countries from last century.
    In NZ many times main stream media conversations quickly start talking about car on car conflict and highway speeds, This is where I need to learn to keep the conversation on the right to have safe access within our local village/town/city, and we are hoping to find a sound bit, and clear statements from central government, NZTA, local government and the Auto Ass. on safe speeds that we can repeat and repeat till it becomes normal to support.

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