Friday, December 04, 2015

A96 in Nairn – 12 years of traffic figures (Part 2 Chewin’ the stat)

In an article earlier this week we produced an image showing Department of Transport stats for traffic point 784 on the map which is the 2.6 miles of road between the B9092 and the A939 junctions (the Ardersier and Grantown turn-offs respectively) . That article is available here and tells you how to download the full statistics in a spreadsheet here. 

Here at the Gurn we are grateful for some thoughts from one of our regular readers. There are those that say stats can be used to say anything you like, perhaps that is a bit harsh but here is what our Gurn think-tank member has to say on these Department of Transport Figures. Click the image below to see the stats.  

Click the image to enlarge 
"Averages can present a false picture – what about peak flow?

Annual averages (which is what these are) even out summer tourist peaks and winter quiet - which a monthly graph would show. Weekly averages would disguise the difference between working-week commuting and weekenders. And - arguably most important of all, even daily averages can be misleading if there is a massive difference between rush hour flows (work and schoolruns), and the quiet inbetween periods of late morning or early afternoon. The point at issue for any road or traffic management is how to cope with peak flow.

Through traffic v local destinations. 

It's a less critical point in terms of the overall picture, but these figures have no way of distinguishing through-traffic from that which starts or ends within Nairn. Of course it all uses the A96. But for planning purposes it will eventually be necessary to have a decent idea on how much of the A96 traffic is through-traffic (since that is your bypass-clientele).

Methodology is the Key

Is this a real, actual count? Is it based on actual totals from a fulltime count of all vehicles (a cable across the road). And is it real data for the whole period of each year? Or are these figures an extrapolation from samples? If we look at the following Department of Transport Road Traffic Count methodology note (available here) we can see that the figures are all estimates. They are the outcome of a selective, limited, "manual" check of traffic on selected "neutral" days, combined with automatic machine-counter readings (cable on road). Then these sample figures are massaged by applying various "expansion factors" and "growth factors" to produce the figures in the table. 

That's not to say they are "wrong". Planning has to use these sort of techniques (at least for forecasts). No doubt the the Department of Transport applies the same methods and factors to all roads and all surveys, so there is at least consistency across the country. They remain estimates. At best a rough indicator. This is not a census. The tables ought therefore to carry a disclaimer or health warning in capital letters.

The impact of the Croy-Cawdor-Auldearn back road "unofficial bypass". 

I can't remember exactly when the sets of additional "Sainsburys" traffic lights were switched on. Was it sometime around 2010? Funny how the total vehicle numbers on this Nairn stretch reached their max in 2010 (12,613) and have declined to the high 11,000-ish in the 4 years recorded since then. To draw any meaningful conclusions from these figures, you would have to do two further exercises:

 (i) look at the figures for the volume of A96 traffic at, say, Hardmuir or Brodie for the same period (where any ratrunning traffic will have rejoined the main A96). If that shows a consistent total figure of 12,600-ish or rising from 2010 onwards, then that captures, and identifies, the scale of the flow which is diverting around Nairn by using the back-road rat-run. Then

 (ii) to test or verify this, you would also have to look at a comparative real-time count (not a sample or a computer model) for that Croy-Cawdor-Auldearn road for the period up to 2010 (or whenever the extra lights went in) and then for the period since 2010. That would - almost certainly - show a rise, and would give a pretty clear measure of the amount of rat-running traffic to compare or correlate with, the "Hardmuir" figures. What chances that Highland Council, which has responsibility for that B-road rat-run, has actually got comparative stats for the traffic volume over the past 8-10 years? The planning conditions and documents attached to the Sainsburys development indicated that the impact of the lights on traffic flows would be monitored by the Council. Has that been done by the Council?"

No comments: