SW Exeter P&R – where will that traffic come from?

At the recent public consultation event on the proposed site, there was a curious display – one that on the face of it should undermine the whole concept of the siting of a park & ride scheme anywhere to the west of the city.

It was this one:

APHINGTON P&R: Origin of traffic on Alphington Road
APHINGTON P&R: Origin of traffic on Alphington Road

Let’s take a closer look at the pie-chart – taken at face value it is suggesting that 60% of the traffic currently using the Alphington Road corridor – and thereby potential users of the new P&R site by the Ide Interchange.

But the chart also raises a number of questions – questions that a statistician would be likely to ask.

Percentages in such diagrams hide a number of sins – where is the total number of vehicles surveyed?

Was it 100? Was it 1000? Or 10,000? and remember that 10,000 would only be about the number of vehicles using Alphington Road in a day.

So was the survey taken over an hour? A day? Or a week?

And how was the data collected?

Was it the result of roadside interviews? Or analysis of data collected by some sort of automatic numberplate recognition?

Earlier data collection has been well documented.

In August 2009, Parsons Brinckerhoff prepared  the Alphington Junction Part and Ride Transport Assessment.

The report contained a table of a survey was conducted by Parsons Brinckerhoff at Matford P&R in June 2003. Users of the P&R were asked the origin of their journey and the results are summarised in the table below.

Origin of Matford users
Origin of Matford users [2003]

In 2004, roadside interviews [RSI] were undertaken at several sites, detailed in the map below.

2004 Roadside Interview Sites
2004 Roadside Interview Sites

These RSI identified the total market for potential Park & Ride users was identified using the origin and destination locations of cars.

This data was used in the Devon County Council’s Alphington Interchange Park and Ride Transport Report published in March 2011.

Origin of traffic from 2004 Road Side Interviews
Origin of traffic from 2004 Road Side Interviews

So is that new pie-chart an outlier? I believe so, and I think the problem stems from WHEN the data was collected.

I’ve already suggested there was a problem presenting undated data, so f my guess is correct, the figures stem from 2014 when Junction 29 was being remodelled and unusual traffic movements would mean drivers were seeking new and unusual ways to get into the city centre.

Whatever the reason for the errors, DCC must stop using this chart if they want to convince people that a P&R scheme is necessary on this site!

UPDATE [24 July 2015]
I have heard from DCC over their definition of East and West.

It appears that those coming from the East aren’t following a star or anything, but from A30 East or from Teignbridge and Plymouth direction. Which in turn means come from the (south) west, via A38 not the east and M5

The data was collected from from Roadside Interviews (RSIs) which are the most accurate surveying method for being able to establish where drivers are starting and ending their journey and therefore what the potential market for Park and Ride may be.

Due to the significant disruption and cost associated with organising these surveys, RSIs are rarely undertaken to support Transport Assessments for planning applications (so in this respect, having the data is a luxury).

There is no intention to undertake any further surveys of this type but I am told  that the Transport Assessment will use Automatic Traffic Count 2014 data.

Generally, the Department for Transport considers traffic data collected within 5 years of the submitted analysis to be suitably representative of conditions

Public consultation events on new proposals for #SWExeter P&R

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Devon County Council and Jacobs are jointly hosting two consultation events on proposals for a Park and Ride site on the South West outskirts of Exeter that will serve the Alphington Road corridor.

Devon County Council has been developing plans for a new preferred site near the A30 interchange junction, between the westbound slip road and the road that leads out towards Ide.

The consultation events will be held at
– West Exe School on Tuesday 21 July, from 4pm to 8pm, and
– Ide Village Hall on Thursday 23 July, from 4pm to 8pm.

County Council officers and Jacobs staff will be present at both events and be on hand to answer any questions.

Alternatively, if you are unable to make either of the two events then the consultation material will be available online at http://new.devon.gov.uk/haveyoursay for three weeks from Tuesday 21 July. The closing date is Monday 17 August.

There is a detailed media release on the Devon News Centre site – Consultation on Park and Ride proposals – which contains quotes from Councillor Andrew Leadbetter [CON, Topsham/St Loyes], Devon County Council Cabinet Member for Economy and Growthm and Cllr Councillor Alan Connett [LD, Exminster/Kenton] 

 

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