Guidance

Evidence about 20mph journey times

This page explains the evidence we used to conclude that ‘Most journeys will be around one minute longer.’

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First published:
23 November 2023
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Details

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. 

Technical response

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.

Data

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
    source: calculation
  • Average speed in a 30mph speed limit taking account of 46.1 seconds of delay per vehicle mile
    value: 22mph
    source: calculation
  • Average speed in a 20mph speed limit taking account of 46.1 seconds of delay per vehicle mile
    value: 19.5mph
    source: calculation
  • 2.7 billion miles/22.2mph
    value: 122 million hours
    source: calculation
  • 2.7 billion miles/19.5mph
    value: 138 million hours
    source: calculation
  • Total additional minutes on the road138 million hours-122 million hours and conversion to minutes
    value: 1 billion minutes
    source: calculation

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

 

  • Total car journeys in Wales, 380 * 3.1 million
    value: 1.2 billion
    source: calculation
     
  • Divide total additional minutes by total car journeys for average change per journey, 1 billion/1.2 billion
    value: 0.85 (~50 seconds)
    source: calculation.

 

 

First published

23 November 2023

Last updated

Part of