Bathampton-Swainswick - Already more than half across the toll bridge?

As in the last two posts, I'm working through the excellent traffic data provided by BathHacked. In this post, I find the data saying that already more than half of vehicles entering Bath at Bathampton and leaving on the by-pass at Swainswick use the toll bridge. Not sure if I believe it yet, but let me know what you think.

 The chord plot showed that

1. short transits of Bath (in and out within 30 minutes) were largely split into separate East-North and South-West groupings.
2. there were about 1,800 such transits in a day past the 'Dry Arch' corner of Bathampton (where the Warminster Road becomes 40mph as you leave/enter the village)
3. The large majority of these Dry Arch transits came from or went to the bypass at Swainswick (eg to the M4).

Taking a two-day sample of ANPR data (31 Oct 17 and 1 Nov 17), the Bath data identify 130,000 individual vehicles. Here's an updated chord diagram, showing that the patterns in the list above are true for a larger 2-day sample.

The BANES Breathe project presented their modelling and plans in the Bathampton Village Hall recently. Many were dubious about the traffic projections for traffic across the toll bridge (and hence through Bathampton High Street). So naturally I want to see what the data say.

I define a 'visit' as a sequence of ANPR readings that starts or ends at the 'radial 1' ANPRs, (See the map) which include Dry Arch, and Swainswick on the by-pass.

Zooming in on visits which start or end at Dry Arch, nearly 11,000 vehicles were observed on such visits in the two days. We know from the data which direction the vehicles were heading, so we can take the traffic from the chord plot and insert more detail.

I (a) drop the 30 minute threshold (b) check where the vehicle went between Swainswick and Bathampton. There are lots of answers to (b), but we focus on whether the vehicle was seen on Warminster Road (near Trossachs Drive, so a little further into Bath) or on London Road (just beyond Cavendish Bridge)

I tried a number of ways to visualise this. A straight ggplot may be true to geography, but is hard to read! A Sankey plot is better.

But there are quite a few very small links, so it's easier to understand if we drop the links that are for fewer vehicles than 0.5% of the total. Then we get this. 

Visits start on the left and end on the right.

To give an example, of the 6,438 visits starting at Dry Arch (the orange rectangle "Bathampton_W"):

• the majority head into town passing Trossachs Drive ("Warm'rRd_W"),
• a smaller pale blue subset is next seen leaving heading North at Swainswick,
• even smaller subsets leave again at Dry Arch ("Bathampton_E"): commuters, shoppers or day-trippers,
• or are picked up at London Road (maybe the ANPR missed them at Trossachs).

The sharp-eyed reader will have noticed more visits coming into WarminsterRd_W than leaving: the omissions are those that go on to the car parks (in which case they may account for more visits leaving WarminsterRd_E than arrive), or leave by other routes, so aren't of interest for the current analysis of Bathampton-Swainswick.

We could get more detail, but we are interested in the pale blues: the hypothesis being that these go over the toll bridge. Of the 6,438 visits starting at Dry Arch, 1,126 (17.5%) might have gone over the toll bridge. This is an upper estimate, which could more precisely be put that we know that 82.5% did not.

There are 996 possible toll bridge crossings in the other direction, making for around 1,000 per day, in total in the two directions. 

This 17.5%, however, is not the percentage we are looking for. Instead we need a more interesting one. Note the share of dark and light blue at Swainswick_N. Of our 2,077 total visits coming in at Dry Arch and leaving at Swainswick, 1,126 have possibly crossed the bridge. So 54% are already crossing the toll bridge.

This is a surprisingly high percentage to me, given 

1. that most lorries cannot use the bridge
2. a toll is a toll
3. the queues at the toll bridge are unpredictable
4. we only had about 1,300 30-minute transits (see chord diagram), so what happens to the other 2077-1300 ~ 750?

So this definitely needs further checking. The fact that the situation is similar in these data in the opposite direction gives some confirmation, but further validation, for example looking at travel times and vehicle types is for another day.

Notes

Implicitly, this analysis excludes

• around 5% where the ANPR did not identify the vehicle (I don't know the technology, but perhaps the number plate was obscured by another vehicle, or was in an unknown format), 
• where only a single ANPR reading was made in the 2 days.

and hat-tip to the networkD3 package authors, and all the other contributors to R