20mph improves traffic flows

20mph limits can actually shorten journey times, rather than making them longer.  New research from Prof Dr Niels Benedikter at the University of Milan confirms 30 km/h (18.5 mph) as the optimal speed limit for minimising delays through traffic congestion.   “…the largest number of cars [that] can pass a given road section [is] about 2,000 cars, at the optimal velocity of 31 km/h. Therefore, citizen’s time wasted due to traffic congestion can be minimized by decreasing urban speed limits to 30 km/h"

The assumption that 20mph limits increase journey times is wrong. Since most urban speeds are well below 20mph owing to congestion and junctions, traffic can flow more freely at 20mph due to:

  1. Reduced braking distances enabling drivers to leave a smaller gap to the vehicle in front;
  2. Increased junction capacity, as drivers are able to pull into slower traffic more easily; and
  3. More people choose to walk, wheel or cycle as streets become safer, reducing congestion.

The benefits of lower speeds limits in urban areas in terms of reducing road danger and emissions and making communities better for walking, wheeling and cycling are well documented. Lowering the speed limits on motorways in the UK, such as the M25, during times of congestion helps to increase capacity. Another perspective from May 2021, which also looked at optimal speeds on a) faster roads and b) other vehicles, is here.

The latest research from Prof Dr Niels Benedikter at the University of Milan confirms earlier studies that capacity increases when speed limits are lowered in urban areas. There are two opposing forces, each of which could lead to increased capacity:

  1. At higher speeds, drivers can pass a point faster; versus
  2. At lower speeds, the safe braking distances between vehicles is lower.

Benedikter’s work was aimed at finding the speed ‘sweet spot’ where the maximum number of cars can pass a specific point in a given time. By using mathematical modelling, he determined this speed to be 31 km/h.

He concluded:

“The lower speed limit makes the flow more continuous, reducing the occurrence of traffic jams, thus leading to lower total travel times even though the maximal speed is reduced. A city with an overall limit of 30 km/h is not only safer, but also more comfortable for drivers because it reduces the amount of time they waste in traffic jams”

Rod King, MBE, Founder of 20's Plenty for Us commented

“We already knew that 20mph limits are safer for everyone. We now know that at peak times they also help drivers get to their destination more quickly. Lower speed limits ease congestion, leading to shorter overall journey times as gaps between vehicles narrow.

20mph helps towns and cities thrive by getting people where they want to be more effectively. Ask your leaders for 20mph.

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