Standardised Speed Distributions

Following on from our opening blog post on Dynamic Assignment, we wanted to share our next modelling methodology. This focuses on standardised speed distributions used in microsimulation models.

As will be the caveat for all of these topics, these are not ‘set-in-stone’ methodologies and it may that there are other approaches out there. However, we wanted to share our Multimodal approach to encourage improvement and provide guidance for those looking to start developing microsimulation models.

When developing a microsimulation model, one of the early steps is to ensure that the desired speed distributions used in the model are appropriate for the area and network modelled.

At Multimodal, we have a template which bases these initial distributions on two key datasets – the Department for Transport’s (DfT) ‘vehicle speed compliance’ tables and Transport for London’s (TfL) speed profiles.

 

DfT Speed Statistics

Using the DfT SPE011 2016 dataset, the following speed profiles have been calculated:

  • 20mph Built Up Roads;

  • 30mph Built Up Roads;

  • National Speed Limit (60mph) – Single Carriageway Road;

  • Motorways (70mph).

The data has been obtained from the following link:

https://www.gov.uk/government/statistical-data-sets/spe01-vehicle-speeds

 

40mph Speed Limit

Due to no data being available for a 40mph speed limit from the SPE011 dataset, a previous DfT dataset  has been utilised - DfT SPE0102 2014 dataset (latest available data).

This data has been obtained from another engineering consultancy, in collaboration on a microsimulation project.

 

Dual Carriageways

Due to no data being available for a 70mph dual carriageway, an archived DfT dataset has been utilised - DfT SPE0111 2009 dataset – Table TRA9906 (latest available data):

The data has been obtained from the following link:

http://webarchive.nationalarchives.gov.uk/20110218142807/http:/dft.gov.uk/pgr/statistics/datatablespublications/tsgb/

 

TfL Speed Profiles

As the DfT datasets provide no information on 10mph or 50mph speed limits, these profiles have been based on the latest TfL VISSIM template that we are aware of (VISSIM Template v5).

An example of the speed profiles calculated is shown below:

Figure 1 - 30mph Speed Profile Examples

 

Don’t’ forget about Localised Speed Profiles

Whilst the DfT and TfL speed profiles provide a good starting point for developing a base microsimulation model, consideration should be given to the use of more localised speed distributions based on ATC data.

We recently had a project where the 40mph speed limit was affecting the journey time validation of the model. All reduced speed areas and priority rules were deemed appropriate, meaning the speed profiles were considered a case for review. We created a new speed profile using the following methodology:

  1. Identify an ATC in a location that would experience ‘free flow’ speeds for the speed limit required
  2. Extract the hours of 0000 – 0600hrs* for either the day or the survey or the neutral days (Tuesday / Wednesday / Thursday) of the week surveyed
  3. Identify the Minimum and Maximum times from the range of data for your speed profile
  4. As a minimum, calculate the 85th percentile (%ile), 50%ile, 25%ile and 75%ile to create the speed profile for VISSIM input

*The 0000 – 0600hr time period was used to allow a ‘free flow’ speed profile to be determined, away from more congested peak periods.

The end result was a better representation of the local speeds and a much improved journey time validation that met WebTAG criteria.

 

Final Comments

The use of DfT speed statistics and TfL speed profiles are useful starting points in developing a base model for calibration and validation.

However, the use of more local speed profiles can play their part in providing more site specific operation and a more representative performance of local conditions.

Finally, remember to check back to the DfT website to ensure that you are using the most up-to-date profiles, as these tend to be updated on a yearly basis.