Wide-area 20mph zones are proliferating, despite sound evidence presented to the DfT that these zones do not improve road safety, but can: (1) actually increase fatal and serious injury risk for vulnerable road users, (2) do increase vehicle emissions, (3) do waste valuable productive time by lengthening every previously 30mph journey by up to 50%, and (4) do impede the progress of emergency service vehicles.
Research carried out for the DfT (see https://www.gov.uk/government/publications/20-mph-speed-limits-on-roads) did not provide statistically significant evidence that wide area 20mph limits reduce vulnerable road user casualties. Joint ABD/ Bridgstock evidence presented to the Scottish Rural, Economic and Connectivity (REC) Committee in February 2019 provided documented evidence of increases in vulnerable road user deaths and serious injuries after the implementation of some 20mph zones, which had replaced previously 30mph ones (https://www.20ssenseless.org/Scottish-Parliament-Submission.pdf).
Around 2% of UK urban adult-, and 0.6% of urban child-, pedestrian casualties are fatalities. The Ashton and Mackay curve (below; from DfT Web Publication No.16 – Relationship between Speed and Risk of Fatal Injury: Pedestrians and Car Occupants, TRL, Sep 2010) tells us that for this to the case, the average impact speed in 30mph zones is already below 20mph
The ABD contends that 20mph zones increase driver distraction and workload through increased need to speedometer-watch. This degrades crucial observation, hazard perception, -response and avoidance skills. In tandem, vulnerable road users have been given the highly inappropriate perception that 20mph zones are “safer”; so they behave more incautiously in them. This is a perfect recipe for vulnerable road user accidents in 20mph zones to involve a 20mph impact speed; while at higher speeds, the less speedometer-obsessed, more situationally-aware vehicular road user has time to brake/take evasive action; thereby reducing impact speed or even altogether avoiding an impact.
Per the schematic for NOx emissions shown below, for vehicles to travel at speeds at or below 20mph requires the selection of lower gears, which means higher engine revolutions and increased emissions. Transport for London’s own data (at http://content.tfl.gov.uk/london-exhaust-emissions-study-developing-a-test-programme.pdf) provides a catalogue (pp.18-32) of curves for various emissions vs average speed by vehicle type scientifically corroborating this fact – which should anyway be blindingly obvious to anyone with even rudimentary automotive knowledge.
(3) Journey Times:
To travel, say, 60 miles at 30mph takes 2 hours. The same journey at 20mph takes 3 hours: 50% longer. The time taken for every previously 30mph journey travelled at 20mph is therefore lengthened by potentially up to 50%. In the year following the imposition of an 80kph blanket speed limit on France’s previously 90kph rural roads, there were no significant casualty reductions (as had been expected by some so-called “road safety” advocates), but the policy did succeed in deflating the French rural economy by 4.4€Bn (see: https://www.thenewspaper.com/news/66/6628.asp). Similar lost productive time arguments clearly also apply to 20mph zones.
(4) Emergency Vehicle Response Times:
It has already been documented (https://injuryprevention.bmj.com/content/9/3/196) that the carpet-bombing of London boroughs with road humps caused more fatalities through increased emergency vehicle response times than the humps had notionally saved. Similar considerations apply to wide-area 20mph zones; many of which are being implemented in areas in which it is impractical for drivers to pull over to let emergency vehicles pass. Likewise, exceeding the speed limit to expedite their progress is no defence against an illegal speed NIP; so highly inadvisable.