As discussed above, there will be links and road corridors on the network where there is no basis from which to derive an estimate update directly.
There are essentially three approaches:
- Do nothing! Leave the estimate as it is until you have a basis for justifying an update
- Update the estimate to the current year using the growth rate from the traffic group
- Update the estimate with a new estimate based on a method (or experience) from the Asset Manager.
The first approach is on the basis that growth and change are difficult to predict. Therefore any change is likely to be without much substance. To a certain extent this is fair. But over a long period time, there will be some adjustment typically to volumes. This may vary from changing land use to just a change in vehicle use patterns.
Growth groups are a vehicle for incorporating this change into the estimates. While it can be argued, growth on an individual road basis may not be valid, it is representative of what is happening over the whole group. Often low volume roads may only have one link that is counted every five years. So in the meantime, the estimates for that road can be adjusted until the count data allows a reset of the estimate based on a count.
The updating in groups also allows the estimates to be refreshed. The Data Quality Index checks from REG recommend estimates are reviewed, as a minimum, annually for arterial routes, three-yearly for collectors and five-yearly for low volume and access road sections. A regular annual update process does provide a regular mechanism to meet these requirements. A more structured layered review process by ONRC category could more closely reflect the minimum guidelines from REG. A suggested programme could be to review arterial routes and a third of the remaining groups/network annually. The issue is remembering annually to undertake the different parts of the network. The advantage of a network wide annual process is it is easier to maintain the habit.
The third option is the approach to updating the estimate based on the recommendation and advice from the asset manager. This may over-ride the growth group-based recommendation where the asset manager knows for that link or road corridor that an alternative value is required. This may be due to a specific network related issue to that link. Alternative, as described above, a recent count has been taken on a link elsewhere on the road giving a basis to update the estimate for other links along that road based on the change that count has signalled.
It is noted that much focus is on the merits and approach to growth. However, growth rates are typically less than 5% on links, with compromises on estimates of upwards of +/- 10% and more. What is important is that our count programme over several years will pick up many of these links, or those links adjacent, and be adjusted to that count data. Those links that slip through this net should be links of little consequence if the count strategy is set up well.
Overall, the recommended approach is using groups and assigning an average growth adjustment on those links left in the grouping not adjusted directly in the previous steps. This can then be over-ridden by a manual adjustment. This could allow for groups of well-established areas or no exit roads being given zero growth. It also allows for those who do not wish to use groups to set up one group for the network! On a more serious note, there is complete flexibility for the user to utilise groups and how they assign growth factors to those groups as and how much they wish. But also note, road groups are very useful in assessing traffic mix, not just volume growth.