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.