It’s possible to work out how the maximum speed affects journey times by using a bit of basic computer simulation to create a very boring car race.
They race along a straight road that’s roughly representative of 1.2 miles of Chiswick High Road, typical of many bigger London roads.
Google Maps tells me that a typical car journey time along this stretch is 6-16 mins in the afternoon, 5-14 mins in the morning, and 6-16 mins on a Saturday.
So taking an average of 10 mins to do this journey means I’m travelling at 1.2/10×60 = 7.2mph, which is pretty close to the average speed across London of 7.4mph.
There’s about 14 sets of lights/crossings on this stretch so about 1 every 150m.
The video below shows a race between three lanes of traffic along a simulated 1.2mile stretch of Chiswick High Road.
The rows of cars are identical and hit identical sets of traffic lights, except for in each lane they have been programmed to drive up to a different speed limit. The top lane is doing 30mph max, the middle one is doing 20mph max and the bottom one is doing 15mph max.
http://videos.futuretransport.info/cars_15_20_30_test_car96.mp4
Now notice the difference in journey time for the cars towards the back (who are representative of everyone except the first few cars who got up early enough to be driving on empty roads).
Measuring the time for the 5th from the back (circled in red on the video).
| Max Speed (mph) | Time to do 1.2 miles (secs) | Average Speed (mph) | Time spent stationary | Time spent accelerating |
| 30 | 616 | 7.1 | 219 | 67 |
| 20 | 703.5 | 6.3 | 227 | 54 |
| 15 | 735 | 6.0 | 212 | 55 |
Halving the speed limit from 30 to 15 reduces the average speed by 1.1mph. Don’t believe us? Watch the video again. There’s a bit of random variation due to traffic light phasing, but if we simulate 100 random traffic light phasings, on average the difference is around 1.1mph.
For a longer journey of say 3 miles (3 miles across London is longer than 2/3 of all journeys), a 26 minute journey in a 30mph zone would take another 4 minutes at 15mph.
But the cost of this to the economy is high, right? Longer journey times will cost businesses money. Maybe. A delivery driver doing 4 hours behind the wheel during an 8 hour shift would see the number of packages they deliver reduced by about 14%.
28% of the delivery cost is the ‘last mile’ (https://www.bringg.com/blog/insights/4-challenges-of-last-mile-delivery-for-ecommerce/).
So a £1 delivery would be around £1 x 0.28 x 0.14 = 4p more in driver and van costs.
However at a 15mph limit, the fuel consumption of their van would be around 2/3 of the amount of they did 30mph everywhere. If they burn 2 gallons a day currently, that would save about £6/day in fuel. If they deliver £6/0.04p = 150 parcels, the delivery cost would work out lower if the speed limit was 15mph.