Traffic mix is often the poor cousin compared to volume when it comes to traffic data. Even in these guidelines you will note much more discussion about the intricacies of assessing volume estimates and how to consider growth changes and other factors. However traffic mix has an important part to play in our monitoring story and is a key input into various data sets.

Two data set examples are pavement loading and freight movement. Pavement loading is a 4th power law so the significance of loading on our pavements and surfacings all comes from the impact of heavy vehicles. Using even a very simple analysis, if our traffic mix underestimates our heavy vehicles by half, we are underestimating loading by a factor of 16. While this may seem a lot a default mix of 3% heavies can be different to the reality of a 10% mix or mor depending on land use and traffic pattens. Secondly, freight volumes are a good measure of the productivity of a region and freight patterns and changes are important for economic and planning inputs.

So, as part of our traffic estimation update process, it is important to include an assessment of traffic mix. Typically volume estimate updates are about growth and therefore change in the number of vehicles. Traffic mix updates are around checking that the proportion of vehicles in different classes is representative of reality. Therefore it does not necessarily change over time as volume does. But often, value populated in the data are default type mixes and not a reflection of the flow, even if those mixes supposedly represent typical values for that function of road or land use. Hence the need for checking and updating traffic mix.

The process is like the volume update, starting with those links from which traffic mix can be assessed from a direct valid count record or history. Secondly, mix can be inferred from adjacent links. Lastly, mix can be inferred from road which the asset manager assesses will have a similar mix to those with a known flow make up.

The recommended method options are detailed below:

• Where there is a history of counts for the link, say at least three counts in the last 6 years, use an average of the last 6 years counts to inform the traffic mix for the link
• If there is one count in the last 4 years, use that count as the basis for the traffic mix for the link.
• If there are counts within the last four years at other links on the road in the same traffic group, then average these to provide the estimate for the link
  • Note, that the asset manager may split the road into sections if the traffic mix would not be consistent and average these sections as appropriate
• If no valid counts for the link on the road/road section, then select a mix based on an assessment of road with a similar profile such as the same traffic group
  • Typically this is where the grouping by road function is useful for traffic mix, as opposed to the subtleties of growth.
• Setting a traffic mix manually based on the experience and knowledge of the asset manager.

As discussed, groups can be set for both the purpose of growth and traffic mix. Traffic mix is an important consideration. Reviews at least every three years are recommended but an annual process as described above will keep on top of most of the issues. If the traffic estimate data has not been assessed for traffic mix currently, the extra effort in resetting traffic mix estimates is very worthwhile. It will benefit the data significantly, but the exercise will also in itself improve the asset managers understanding of the network.