A DPF filter is designed to collect soot at all vehicle speeds. It still operates in an urban environment with either a 20mph or 30mph limit, collecting soot and depositing it within the filter. However the filter relies upon a process of “passive regeneration” to periodically burn off the collected soot. This would normally happen if the vehicle is running at a sustained speed of 40mph or above. Hence current urban environments with a 30mph limit would not provide the opportunity for such “passive regeneration”. In such cases the engine is designed to automatically trigger a process called “active regeneration” at a pre-determined level of soot collection (normally 45%). In this extra fuel is injected into the engine automatically to raise the temperature in the filter and burn off the collected soot. This process burns off the soot already collected in the filter. This process is already used for vehicles operating in urban 30mph environments.

Hence a 20mph limit will not create any difference in the way that a Diesel Particulate Filter clears itself. A 20mph environment will be no more of a problem than a 30mph environment. If the vehicle is not periodically driven above 40mph then it will simply initiate its own "active regeneration". 

If we look at the few countries that currently use mph for speed limits then certainly Wales will be the first to do so. However many countries using km/h, where the equivalent is 30km/h, have this as a norm for most urban/village roads. Spain has taken a step similar to Wales when it set a national 30km/h limit for any urban/village roads with a lane in each direction to 30km/h in 2021. This is reduced to 20km/h (12mph) if there is no pavement.

Wales will be the first nation within the UK to set a national 20mph limit, however Scotland plans for 20mph to be the norm by 2025.

Already 18 million people in England live in local authorities who have rejected 30mph as an urban/village norm and have set 20mph for most such roads. This includes all the inner London boroughs, most of the large cities and more rural counties such as Lancashire and Cornwall. See our full list of 20mph places.

• Whilst there may be some increase in journey times these are minimal and for most journeys will be less than 2 minutes.
• Yes, there will be substantial benefits in reducing road casualties .
• There will be other benefits from reduced noise, pollution and fuel consumption, as well as positive public health outcomes.

The Welsh Government has published a Regulatory Impact Assesment (RIA) report which sought to forecast the benefits and disbenefits from the 20mph policy. This was made available to Members of Senedd.

The primary factors in this are the potential for increased journey times, fewer casualties and better public health outcomes. On balance we believe that there will be large benefits to the Welsh economy, but the difficult part is not only in estimating the outcome, but also in the quantifying that as a benefit or disbenefit. This difficulty was referenced in the RIA, which concluded that an "indicative forecast" of the Net Present Value of benefits over 30 years were between +£1,870mn (excluding journey time) and -£4,480m (including journey time). There were a number of caveats to this which explained the range of forecasts. The figures shown are in £ millions as a Net Present Value over 30 years with positive figures being a benefit and negative a disbenefit. All following information is from the report except that asterisked and indented. :-

Direct Capital Costs (-£32.5m)

These were predictable and all funding is provided by the Welsh Government. Although local authorities are implementing the changes there is no net cost to them.

Increase in Journey Times (between 0 and -£6,350m)

Three quarters of the disbenefits are associated with trips with time impacts of less than 2 minutes. In addition, only 26% of these trips were associated with business and 74% were either commuting or leisure. With average journey time increases being less than a minute and most journeys affected by less than 2 minutes there is great uncertainty about the opportunity cost of that time, especially with such a high proportion not being business mileage. And the methodology dictated for use in such a calculation was required to use the formula developed for major road construction rather than minor changes to short journeys in urban settings. It also does not allow any reduction on cost for commuting/leisure compared to business. For these reasons the RIA decided to include the NPV excluding journey time which then resulted in the +£1,870 net economic gain. 

Road Casualty Reduction (between +£479m and +£2,533m)

These were calculated by using the forecast reduction in speed and applying factor which predicted the reduction in casualties. This varied from a low of preventing 1 fatality, 62 serious and 198 slight injuries pa annum to a high of preventing 15 fatalities, 130 serious and 299 slight injuries per annum. When using a standard DfT formula for the value of casualty prevention this amounted to the NPV of the resulting benefit being between £479 and £2,533m. 

* A separate report by the Transport Research Institute of Edinburgh Napier University predicted a value of the prevention of casualties at £92m per annum. This would confirm the higher benefit from the RIA. Note that much of this would benefit the NHS in reduced A & E resources required to attend to road casualties..

Increase in Cycling and Walking (between +190m and +£948m)

The RIA has based the benefits from walking and cycling on 4-6 extra cycling trips per person per year and 1-2 extra walking trips per year attributable to the 20mph policy by 2040. This mainly comprises the benefits to public health from the active travel increases associated with a calmer and safer urban and village streets.

* Separately we note that the annual cost of obesity in Wales of £3,000m per annum so even a 2% reduction from increased active travel would result in a £60m gain per annum.

Reduced emissions (between +£2m and +£8m)

A small environmental improvement resulting from lower non-exhaust particulate (PM2 and PM10) emissions can be expected from the policy. Lower driving speeds should result in lower tyre, road and brake abrasion reducing particulate pollution in communities. We note that this benefit is somewhat but not entirely offset by increased exhaust emissions of Nitrogen Dioxide (NO2) from fossil fuel powered vehicles at lower speeds.

* Note that more recent research is showing substantial reductions in emissions when in a congested stop/start environment. In such circumstance the repeated acceleration up to a speed limit has a far greater impact that steady-state driving. This has shown that when congestion, junctions, lights, crossing, require stopping at a frequency of 220m then a 25% reduction in emissions would result. This would result in a large increase in emission reduction and benefit to the above figures.

Other benefits not taken into account

The RIA notes that there are several benefits that have not been taken into account. These include :-

• Improved functioning of the road network / smoother traffic flow / reduced congestion as a consequence of slower speeds. For example, the ability of vehicles to move in and out of parking bays or change lanes more efficiently at lower speeds.
• While many benefits from increased active travel have been included such as reduced mortality and decongestion. Broader benefits such as the day to day benefit of increased physical and mental health from increased exercise remain unquantified. Furthermore if increased levels of active travel disproportionally remove car trips that affect stop-start
  congestion during peak periods such as school runs , the decongestion benefits from increased active travel could be larger.
• Noise: it was considered that the evidence base was not conclusive enough to form a monetary judgment for the policy. However, we note that the Welsh Government has
  recently commissioned additional research and analysis of likely noise-related impacts of the policy, to be performed pre- and post-policy implementation, to gain a better
  understanding of the relevant impacts.
• Increased retail spending, resulting from improved access (particularly amongst those with mobility challenges). At this point in time however good quantitative evidence on which to calibrate this benefit remains lacking.
• Increased property values (and the resultant land value uplift) are also not included as a credible methodology could not be assembled to monetise these benefits given available data and pre-existing studies, and available resources.
• Once the 20mph policy has been fully implemented, it is expected that a wider range of data will become available. This will enable an analysis of the policy’s broader impacts,
  including in relation to the aspects outlined above. (The proposed monitoring of the policy’s impacts post-implementation is discussed in section 7 of this RIA).

* In addition, we identified several other benefits that were not included :-

• The same research on urban emissions also calculated that under the same congested conditions drivers would save up to 30% of their fuel usage (up to 10p per mile) where a 20mph limit was in operation compared to a 30mph limit. 
• In just the same way that casualties will be reduced because of reduced speeds, so will non-injury collisions. This will be a benefit that will be reflected in both reduced costs of such collisions to drivers involved and their insurers.
• Many other initiatives to support active travel become cheaper and more cost-effective in a 20mph environment compared to 30mph.
• Tourism can be boosted by visitors from other countries where they drive on the right feeling much more comfortable in a slower speed environment.

20's Plenty believe that the benefits from the 20mph policy are large and will benefit the Welsh economy.

Rod King MBE, Founder and Campaign Director commented :-

"In particular the idea that it will "cost" the Welsh economy £4.5BN is predicated on outdated UK treasury orthodoxy in accounting. There is no "cost to the economy" of either setting off for work or school a minute earlier or arriving home a minute later. For this to be true then lingering over a coffee whilst shopping or stopping to look in a shop window whilst walking to work would equally impact the economy. It is clear "bunkum". Wales and its' economy will gain hugely from the adoption of its national 20mph default."