If they put up a ton of 30 signs then it probably would be alright. Eg, the main road through a village or town could stay 30, while the main high street and side roads would all be default 20. But that requires more than just a change in law to say “what was 30 is now 20”, and they don’t seem to be doing this. They’re expecting financially strapped local councils to go through a process of assessing and assigning 30 limits themselves, at their own expense. It likely won’t happen in most places, they simply can’t afford it.

Also, if you want to refer to that blog post as a “study”, we should look only at its sourced claims.

1. Motor traffic volumes decrease since slower speeds encourage active, sustainable and shared travel. Walking and cycling levels rose by up to 12% after Bristol’s 20mph limit ^[1]

This is specifically about Bristol, a city with narrow roads.

2. Smoother driving with less wasteful braking and acceleration cut fuel use by 12% in Germany after 30kmph (18.6 mph) limits were implemented ^[2]

I’m sure Germany have implemented speed limits efficiently, but the UK has a history of compelling local councils to implement traffic measures that increase fuel consumption and thereby increase fuel tax revenue. Regardless, Germany have not changed all of their 50 kph zones to 30 kph, like this law proposes.

3. The Department for Transport’s speed / flow plots are in the COBA (Cost Benefit Analysis) manual - Vol 13 of the Design Manual for Roads and Bridges. This computer programme shows that urban traffic flow improves at lower speeds ^[3]

Urban traffic flow. This law covers everywhere.

4. The FREEFLOW project by the University of York, City of York Council and others… Slough experimented with this successfully in the 1950s on a single arterial road. ^[4]

This experiment was from 1950.

5. Shared Space also enjoys claims of shorter tailbacks and congestion. The concept relies on cutting traffic speeds to around 20mph or less for ‘eye contact’. Traffic experts Ben Hamilton Baillie and Phil Jones state “tailbacks of traffic during peak periods have also reduced. It seems that the ambiguous junction provides improved capacity for traffic and fewer delays than traffic signal control systems.” ^[5]

Urban design, referring to handling specific zones of congestion.

None of these points apply to changing the national speed limit for Wales from 30 to 20.

Also check out this fancy markdown citation function!