Lowering speed limits on British roads had essentially no effect on the average speeds of motorists. That was the finding of a study released last week by the UK Department for Transport (DfT). The DfT has touted lowered speed limits as a means of forcing traffic to slow down, thereby increasing safety. The results showed, however, that dropping the limit by 10 MPH failed to reduce speeds by 10 MPH.

"Journey speed analysis shows that the median speed has fallen by 0.7 MPH in residential areas and 0.9 MPH in city center areas," the report found. "There was no significant change in three case study areas."

Even that conclusion must be qualified. Speeds also fell slightly at control sites that kept 30 MPH limits, so the statistical analysis showed the relative change in median speed was actually 0.38 MPH on all roads with the newly lowered limit.

"The scale of the change is expected as a substantial proportion were already traveling less than 20 MPH before the new limits were introduced," the report noted. "It appears that the nature of the roads where the limits have been introduced means that lower speeds were already self-enforced."

The researchers evaluated a dozen locations where the limit was lowered from 30 MPH to 20 MPH across more than 400 miles of road. Up to four years of data after the change was implemented was compared with three control locations where the speed limits stayed the same. The GPS navigation company TomTom provided speed data, and spot checks with radar guns supplemented these figures.

Speed limit lowering has not been universally accepted. One of the comparison areas, Worthing, held a referendum on the the issue in 2014. Residents rejected the scheme with 69 percent of the vote. Nonetheless, UK officials continued wide imposition of 20 MPH limits with local authorities in 2013 being given discretion to lower limits, supported by pressure from special interest groups promoting the change.

"A vocal and active pro-scheme campaign group... was a key enabler in three schemes, and the most important factor in one scheme," the report noted.

That pressure group, Twenty's Plenty For Us, blasted the new research. Rod King, founder of Twenty's Plenty, insisted the findings were statistically invalid.

"The 20 MPH research study from DfT was supposed to measure whether mean speeds had reduced," King wrote Monday on Twitter. "However it used 'median' rather than 'mean' speeds. It comments, 'This helps dampen the impact of slow moving vehicles.' I don't find that credible. Do you?"

If the questionnaires sent to residents in the areas where limits were lowered are accurate, only 22 percent agreed with King that the new speed limit signs have cut speeds. By far, the most significant change in the residential areas where 20 MPH was implemented is that compliance fell from 91 percent to 47 percent, which enables an increase in speeding tickets. Several of the pressure groups behind the lowering campaigns have a financial interest in that outcome.

The UK government funded several campaign groups involved in lowering speed limits, as a report from the Taxpayers Alliance documented (view report). Other promoters have included insurance companies like the RAC, which see an increase in revenue from license points for every speeding ticket issued. The pressure groups are calling for an increase in ticketing operations and speed cameras, citing the "lack of enforcement" of the 20 MPH limit.

"In general, fixed speed cameras have not been used in the case study areas," the report explained. "This is partly due to the cost of buying and maintaining the equipment; but also because they are seen as being most appropriate for enforcing limits in casualty-led scenarios. Most of the 20 MPH limits in the case study schemes have low casualty rates, and the use of fixed speed cameras is expected to be contentious."

The analysis showed there was no statistically significant change in the accident rate after the limit was lowered.

A copy of the report is available in a 4mb PDF file at the source link below.