20’s Plenty is often asked if limiting most of a built up area road network to 20mph somehow diminishes the focus (e.g. funding or driver attention) away from places that are especially dangerous – such as those with a history of casualties or where children congregate?  This question often comes from road safety professionals.  Turning most roads 20mph could seem a simplistic response to road danger if there are hot spots of clustered casualties, or vulnerable people, that apparently warrant relatively expensive road layout or engineering changes.   Continue reading

In December 2017, various newspapers published articles with variations of "Streets where more people died AFTER 20mph limit came in ...but councillors say it's too expensive to scrap it". They had picked up on an article in the Bath Chronicle about a report submitted by a Bath and North East Somerset Council officer to their Scrutiny Committee in July 2017. In fact the committee rejected the report and took no action as a result of it. In fact the report did NOT include any details of "deaths" and only compared "killer or seriously injured" across various wards. In doing so it was comparing very small numbers such as 0.8 and 0,6 for previous and after periods. Any statistician will tell you that such comparisons have no statistical significance and are heavily influence by the randomness of crashes rather than representing any trends. However we did a thorough analysis of the results in the report and these show that :- Continue reading

Often we are asked if a 2-3mph change in average speeds is worthwhile when you reduce the limit from 30mph to 20mph. To understand this and how it can produce reductions in casualties of 20% or more we have to understand that these are 20mph limit implemented across a wide area. These range from slow and quiet roads where previous speeds already are less than 20mph, through roads with speeds of 20 to 24mph and even some where speeds are between 25 and 30mph. Besides being a wide range of speeds beforehand there is also a wide range of casualty history. Many of the lower speed roads will have minimal casualties whilst faster roads may well have significant numbers of casualties.. Continue reading

Many people think that driving at 20mph lowers fuel economy. Yet research shows that for many cars a steady 20mph is the most efficient speed if you can keep it steady. Indeed at 20mph many cars will achieve fuel consumption better than 90mpg. But what changes things is in an urban environment where you are consistently slowing down for obstacles, lights, junction and congestion and then having to speed up again.   Here we explore the engineering principle behind the energy that gets used by a motor vehicle when moving. Continue reading