Sustained high speeds along the Warminster Rd, Bath

In the last few posts, I was analysing number plate reader (ANPR) data from B&NES Council and BathHacked, a data activist organisation, looking at how traffic patterns might change if a clean air zone were introduced.

I've gone off on a tangent, analysing data from just two ANPRs, a mile apart on the A36 Warminster Rd, on the outskirts of Bath. They give me average speeds for some 65,000 transits over two weeks.

This is a residential 30mph zone, with a straight section, blind bend and narrow hill, though perhaps you might not think that, judging from this box plot (a smaller sample of 3 days). Each dot is a vehicle along the road. The lowest speeds are where they stop or detour (I'll put a lower limit of 5mph on to eliminate these). November 8 & 9 saw a little queueing heading into town in the morning, but these were the only cases where the speeds are not clustered around 30mph. (That's "around" - little sign of a limit in these data.)

From the data, we can split vehicles into cars, light commercial (LCVs), heavy commercial (HCVs - this is a trunk route with large lorries), public-service vehicles (PSVs) and a rare few others that we'll ignore. The scary thing is that there's no sign that trucks are travelling any more slowly than cars. Only public-service vehicles are noticeably slower, given the 3 bus stops on this section. (All days of data are included here.)

In this large survey, at peak hours there were typically 40 vehicles per hour sustaining 33mph or more over the 1 mile route. The top sustained speed was over 60mph, and one van driver managed to get 3 times into the top 20, each time doing more than 50mph.

Definitely some room for improving safety here.

Code is available on github, data from BathHacked.org.

How much traffic might switch to the Toll Bridge to avoid the Bath CAZ?

A high estimate, based on 2017 traffic counts of A36-A46 transits, is that up to 30 vehicles/hour on this route could avoid the Bath Clean Air Zone (CAZ) and join the evening peak traffic (currently 100/hour) on the Toll Bridge. This is a 'high estimate' because it (a) ignores 'retirements' of old cars over the next 3 years (b) assumes all vehicles that currently go through Bath, are liable for the CAZ and could switch to the Toll Bridge, would definitely switch to avoid the CAZ.
In previous posts, I've explored the excellent Bath traffic data provided by BathHacked, looking at transits of Bath (which split into two groups North-East and South-West), and transits using the entry/exit points on the by-pass at Swainswick, and the A36 at Bathampton (Dry Arch). 

Now, I finally zoom in to get a first answer to the question of how much more traffic might there be over the Toll Bridge when the CAZ starts. I'm using (just) 2 days of data 31 Oct 2017 and 1 Nov 2017.

From the analysis so far, the main flow likely to generate additional traffic over the Toll Bridge is A36-A46 transiting traffic. I've not shown it so far in the blogs, but I found little evidence in the ANPR data of significant traffic on local flows with a choice between BathwickRoad-LondonRoad and WarminsterRd-TollBridge. There must be Batheaston-Combe Down traffic, for example, but not large compared to the 1,000 per day each way on the A36-A46.

First, here are the A36-A46 flows. In these diagrams traffic, like time, is going from left to right: start at Swainswick heading South, and leave at Bathampton heading East; or vice versa Bathampton West then Swainswick North. The height represents the number of vehicles: the thin 'Swainswick_S' vertical green bar represents around 2,600 (south-bound) transits.

Under half of the vehicles on these transits are detected by the ANPRs on London Road, or Warminster Road. While a few vehicles' number plates might not have been read at either intermediate point, it seems a reasonable assumption that most of the remainder (the light blue) simply took the 'direct' route avoiding the intermediate ANPRs, over the Toll Bridge. 



The key question is - how many vehicles currently go through Bath rather than take the Toll Bridge now, which might switch when the CAZ comes in?

To answer this, I first plot the traffic by hour of the day. We've seen that slightly more than half already take the bridge. The bar chart below shows this again, and shows in addition that the bridge traffic is fairly steady through the day (remember this is A36-A46 transits, so won't include school runs), while through Bath there's a more pronounced mid-day peak: less transit traffic in the morning and evening rush, for obvious reasons!

The chart has 2,606 vehicles over the Toll Bridge, 1,983 going through Bath in total. The peak hours on these two days for transits using the bridge are the evening, around 100 vehicles per hour (total in the two directions). 

Additionally, the chart shows the split between petrol, diesel and hybrid or pure electric - though this difference isn't essential here.




Then I split the vehicles into:

• heavy-commercials (HCVs), which nearly all go through Bath, because they have no choice;
• those with high or unknown emissions and therefore likely to be subject to the CAZ charge;
• those with low emissions and therefore definitely exempt.

If the CAZ came in today, the vehicles that might change their behaviour are the middle set on the right: those that currently go through Bath and are likely to be liable to the charge. The proportion of these changing is unlikely to be 100%, because some might choose to go through Bath for other reasons (dropping someone off, say), or it might just be that we've classified the car as 'unknown' when they would not be liable, so an estimate here is an upper bound. Let's assume it's 100% for now.

The graph shows that, in the evening peak there's up to 30 cars per hour that could be tempted to make the switch, if the CAZ happened today. There's a little over 30 in the middle of the day, but the bridge is slightly quieter then.

But these are 2017 data. The key is assumptions about 'retirement' of the vehicles between now and the start of the CAZ at the end of 2020. Unfortunately, for the moment I haven't found good data to estimate that retirement rate, though my recollection is that the BreAthe consultation suggested retirement rates of 70-80%. That would bring us to the magic number of 'around 10/hour', but I'm not currently in a position to explore that step further.

More evidence that most cars and light commercials transiting Bath (Bathampton-Swainswick) already use the toll bridge

In an earlier post, I estimated that nearly half of vehicles transiting Bath between the A36 Warminster Road and the By-pass at Swainswick already used the toll bridge.

In this post, I provide more supporting evidence for this: showing that the split by vehicle types makes sense.

Nearly all heavy commercial vehicles (HCVs) go into town (via Warminster Road and London Road, or vice versa) and then back out again. 

Recall that in these plots, a transit of Bath starts on the left and ends on the right. There's just the thinnest of light blue lines at the top of the diagram for HCVs which are not recorded by ANPRs between Swainswick (south-bound) and Bathampton (Dry Arch). 



The majority of cars and light commercial vehicles that are heading South (see the "Swainswick_S" node on the left) are next recorded at "Bathampton_E" (Dry Arch, leaving Bath). These, we are assuming, use the toll bridge, although a small percentage might have been missed by 2 ANPRs on the way (previously we estimated this 'missed' rate at less than 5%).

Heading North, the proportions are slightly smaller, between 50% and 60%. 

Nearly 30% of heavy commercial vehicles transiting Bath pass through Bathampton (on the A36).

We looked in earlier blogs at the overall patterns of traffic transiting Bath, using council and BathHacked data. 

In this blog we drill into the detail by type of vehicle, essentially car, and light- or heavy-commercial vehicle. We show that more transiting heavy commercial vehicles go through Bathampton on the A36 than use the Batheaston dual-carriageway.

For variety, and to show it's not very sensitive to the time threshold, in this blog we use a 45 minute threshold, and a 2-day sample (31 October and 1 November 2017). The chord diagram, which is aligned roughly geographically, looks like this.

Splitting each chord by type of vehicle, and colouring them not by starting point, but by type of vehicle gives this. Although it shows that cars are by far the biggest number of vehicles, it's hard from this to see if there are differences in the patterns.

A diagram for each could be confusing because the scales are now different: total in and out at Bathampton (Dry Arch) is near 2,000 for cars, 700 for light, and 450 for heavy commercial vehicles. 

Cars and light commercial vehicles have quite similar patterns: for light commercials there's perhaps a little higher proportion on the Swainswick-East link. But heavy commercials are very different:

• a much higher proportion are really crossing town: Bathampton - West;
• there's a much smaller proportion of traffic South-South, or South-West.

As a result, Bathampton has more than twice the share of heavy commercial transits than its share of total transits. 12.5% of all transits (3003/23841 in our sample go in or out at Bathampton (Dry Arch), compared to 28.5% (318/1116) of heavy commercials.

In fact, more transiting heavy commercial vehicles go through Bathampton on the A36 than use the Batheaston dual-carriageway.

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

Bathampton - transiting traffic

Building on the previous blog post, I've extracted some 13,000 vehicle transits of Bath on 31 October 2017 from data provided by BathHacked. (These are all the entries and exit again within 30 minutes, through the 'radial 1' on the map shown before.)

A chord plot, using the great circlize package in R is a good way to show these flows. The counts around the edge are transit entries or exits - so the total is 26,000. Direction is indicated by the slight point/arrowhead.

You can see a big influence of the Batheaston bypass in the flow between North (Lansdown and Swainswick (dual carriageway)) and East (Box Road & Bathford). That's expected.

But I'm really surprised that the very large majority of vehicles entering or leaving Bathampton (near Dry Arch) and transiting Bath are also heading to, or coming from the North. Around 700 heading North, and 1,000 heading South. Very little traffic transiting and heading West, for example.

Can this be correct? More checking needed, I think.

(Note that one vehicle might generate more than one transit in the day

Bath - location of ANPRs

I didn't manage to make it to the Bath Hacked day on analysing road traffic data, but I'd still like to investigate a little. Big shout to the Bathhacked folks for their work!

First off, a quick visualisation of the locations of the automated number plate readers (ANPRs). I did this in Tableau, since that's quickest for me. If you don't know Bath, sorry that it's a bit cryptic, but if you do, I suspect it gives enough information. Might be better with a bit of road information behind, but that would take longer!

The ANPRs mostly come in in-out pairs, and in 4 "radial groups": basically one a long way out, then 3 concentric circles. They're also identified by direction of travel, NESW, but given what I have in mind, this is less interesting so I didn't show it.