We understand that some public concerns about the introduction of the 20 miles per hour (mph) speed limits relate to the statement made by the Welsh Government; ‘Most journeys will be around one minute longer.’
This statement is supported by detailed technical assessment that considers the population, car journeys and distances travelled in Wales. This shows that on average, each car journey will be one minute longer because of the change in speed limits to 20mph. The types of trips made on the affected roads are typically short and involve stop-start travel conditions. This means that the actual impact of the change in speed limits is often marginal.
We understand that some public concerns about the introduction of the 20mph speed limits relate to the statement made by the Welsh Government; ‘Most journeys will be around one minute longer.’
This statement is supported by detailed technical assessment informing the Regulatory Impact Assessment (RIA) that considers a complex range of inputs and outputs associated with relevant open-source data pertinent to population, car journeys and car Kilometres (Kms) in Wales.
Notwithstanding the explanatory text provided as part of the RIA including but not limited to that referenced on pages 6, 26, 29 and 41 regarding journey times, some of the calculations that informed the RIA are summarised below to help explain the assumptions behind the statement.
Road traffic estimates published by Welsh Government show that there were 25.1 billion car Kms driven on Welsh Roads in 2019. 4.4 billion Kms (17.5%) are on roads that are expected to experience the change to a 20mph speed limit.
Most trips on these roads are short in duration, for example to school, local shops or to visit nearby family members and friends.
With the shift to a 20mph speed limit, car Kms on these roads will see longer travel times. This has been calculated referencing Office of National Statistics (ONS) data including average free flow speeds on 30mph and 20mph roads, which are 31mph and 26mph respectively, experiencing an average delay of 46 seconds per mile.
Average speeds in typical driving conditions are therefore 22.2mph on 30mph roads and 19.5mph on 20mph roads. The reason for this slight difference is largely because free flow traffic generally exceeds the speed limit in 20mph zones. The difference in typical journey speed, 2.7mph, is then applied to the 4.4 billion car Kms to provide a value of 1 billion additional minutes per year.
To estimate the time change per journey, the total number of car journeys is estimated by multiplying the average number of car journeys per person per year, 380 (National Travel Survey (NTS) data), by the population of Wales, 3.1 million, which equals 1.2 billion car journeys by Welsh residents per year.
The division of 1 billion additional minutes per year by 1.2 billion car journeys results in an average increase of 50 seconds per journey, or when rounded to provide a reasonable worst case; of around 1 minute.
First need to understand the total amount of additional time spent on the road due to the change in the speed limit
- Total Kms made by car journeys in Wales in 2019
value: 24.9 billion
source: Table 1, Road Traffic 2019
- Total Kms made by car journeys in Wales in 2019 on roads expected to see change to 20mph speed limit
value; 4.4 billion
source: calculation based on roads affected and share of total 24.9bn on these roads. (Table A5.5 in Regulatory Impact Assessment)
- 4.4bn Kms conversion to miles
value: 2.7 billion
- Average free flow speed in a 30mph speed limit
source: Figure 11, Vehicle speed compliance statistics for Great Britain 2021
- Average free flow speed in a 20mph speed limit
source: Figure 11, Vehicle speed compliance statstics for Great Britain 2021
- Average delay to car journeys under typical driving conditions and congestion compared to free flow
value: 46.1 seconds per vehicle mile
source: Figure 2, Travel time measures for local ‘A’ roads: January to December 2021 report
- Average speed in a 30mph speed limit taking account of 46.1 seconds of delay per vehicle mile
- Average speed in a 20mph speed limit taking account of 46.1 seconds of delay per vehicle mile
- 2.7 billion miles/22.2mph
value: 122 million hours
- 2.7 billion miles/19.5mph
value: 138 million hours
- Total additional minutes on the road138 million hours-122 million hours and conversion to minutes
value: 1 billion minutes
Then need to understand the total number of journeys made by car to divide the total time by total journeys to estimate average change per car journey
- Car journeys per person per year
source:National Travel Survey: 2020
- Population of Wales
value: 3.1 million
source:Population and household estimates, Wales: Census 2021
- Total car journeys in Wales, 380 * 3.1 million
value: 1.2 billion
- Divide total additional minutes by total car journeys for average change per journey, 1 billion/1.2 billion
value: 0.85 (~50 seconds)