TfL Ref: FOI-0673-2223
Thank you for your request received by Transport for London (TfL) on 17 June 2022, asking for information about speed surveys.
Your request has been considered in accordance with the requirements of the Freedom of Information Act and our information access policy. I can confirm that we hold some of the information you require. You asked:
If for any roads that have had their speed limits dropped from 40 to 30mph in the last 15 years and speed surveys were conducted before and after the change. For each road what changes were recorded to the following
The average speed
The 85th percentile speed
The average speed of the fastest driver ie the faster 1 or 5%
The overall level of compliance with the speed limit.
Is the methodology designed to record free-flowing traffic speeds, so it can't be skewed traffic or by a group of cars following a single slow vehicle etc?
The Lowering Speed Limits programme in TfL was initiated in 2018. This programme covers only the Transport for London Road network (TLRN), not roads managed by the London boroughs or other Highway authorities. Details of the Lowering Speed Limits programme and future proposals to reduce the speed limit to 30mph can be found at https://haveyoursay.tfl.gov.uk/lowering-speed-limits.
Under the Lowering Speed Limits programme there has been one project where the speed limit was reduced to 30mph from 40mph: A10 Great Cambridge Road between Great Cambridge Roundabout and White Hart Lane in the London Boroughs of Enfield and Haringey.
The attached speed surveys were obtained prior to the speed limit reduction in October 2020. The output of this, provided by Tracsis plc, shows the average speed, 85th percentile and 95th percentile speeds by direction across the three sites. It also shows the proportion of the traffic flow travelling above the posted speed limit, as requested, as well as the proportion travelling above the Association of Police Officers (ACPO) and above the Department for Transport thresholds.
We intend to commission comparative surveys for the post-implementation period for October 2022 so that we can attain seasonal matching.
Regarding the methodology used to record free-flow traffic, in London we use the 7pm-7am speed data as a proxy for free-flow traffic conditions, which was agreed with the Metropolitan Police. Attached is the DfT methodology for calculating vehicle speeds. When this methodology was applied to the TLRN as part of TfL’s 20mph trial programme, over 85% of the results were excluded from the sample. Consequently, it was agreed with the Metropolitan Police to adopt the 7pm-7am proxy for free-flow conditions on London’s roads.
Please note any other projects which have lowered the speed limit to 30mph schemes on the TLRN due to safety or operational reasons, outside of the Lowering Speed Limit programme, aren’t included in this response. In order to provide you with this information, you would have to identify the locations you are interested in. As unfortunately, to gather all this information would exceed the ‘appropriate limit’ of £450 set by the Freedom of Information (Appropriate Limit and Fees) Regulations 2004. Under section 12 of the FOI Act, we are not obliged to comply with a request if we estimate that the cost of determining whether we hold the information, locating and retrieving it and extracting it from other information would exceed the appropriate limit. This is calculated at £25 per hour for every hour spent on the activities described.
If this is not the information you are looking for, or if you are unable to access it for any reason, please do not hesitate to contact me.
Please see the attached information sheet for details of your right to appeal as well as information on copyright and what to do if you would like to re-use any of the information we have disclosed.
FOI Case Management Team
Transport for LondonTfL Ref: FOI-0673-2223