Friday, 20 September 2013

Peter Abraham: show us the 20 mph road rage data

One goal of Bristol Traffic is to create datasets on how people get round bristol. It had been to create a  ubiquitous mass surveillance police state -but it turns out Google have done that and then pass on the details to the NSA. So a focus on local issues for us.

While covering the Down's refusal to adopt 20 mph speed limits,we picked up on a statement by Cllr Peter Abraham:
A 20 mph limit might be appropriate for some streets, but I think it will cause road rage incidents and a lot of frustration for motorists on The Downs."

This assertion, road rage causes frustration is news to us. We've been against 20mph for a long time, not because it increases the likelihood of us getting out a cricket bat and bashing in the windows of the Citron Dyane doing 19 mph in front of us, but because it means it will take longer for us to get round the city.

Now  presumably road-range incidents need >1 vehicle, so driving at 20 mph when it is quiet isn't going to cause you to jump out the car, get out the old cricket bat and bash in your own windscreen. The probability of a road rage incident must not only be a function of the driver, the road and the effective limit, but of the speed of the vehicle in front:

For any driver, Dn the probability equation becomes one of:

P(road-rage(Dn, road)) = fn(driver, road, limit, velocity(Dn), velocity(Dn-1))

For an entire road over a day, with K drivers, the probability of a road-rage event is

Sum(Dn=1..K) P(road-rage(Dn))

To predict that road-rage frequency, we need to know the value of the function fn(driver, road, limit, velocity(Dn); we can derive the rest from there.

It is critical we get the numbers to derive this, as well as understand where it is a discrete function such that it returns 0 for all speed limits >20, or if some drivers are capable of going over the edge at 30 mph, 40 mph, etc.

Nobody appears to have this data other thal Cllr Abraham, who is shaping city speed limits based on his private dataset.

It's more important than the downs too. If the road-rage probability gets higher whenever the speed of the vehicle in front is less than 20 mph,  velocity(Dn-1)<20), then anything in our city which forces people to drive below 20 mph is going to create incidents.

That means we have to ban bicycles from all city streets where a car could be held up behind them.

For example, Hampton Road, redland. There would be a nice fast 30 mph stretch between two traffic jams, yet WV06WML is trapped in traffic between two bicycles. He only just manages to clear one and so sprint up the hill to the St Michael's Hill roundabout traffic jam by aggressively overtaking it and swinging in before hitting the oncoming cyclist.

if there were no cyclists here this incident would not have arisen. Admittedly, the journey time of the car would be the same -it's a different function, one that depends on the congestion of junctions, so is a function of all road users in the city at a specific time, not just the velocity of the vehicle in front.  But we aren't worried about that: we are worried about road rage.

Peter Abraham appears to be the only person in a position of authority in Bristol with the confidence to assert that 20 mph limits increase road rage. He needs to publish this data, not just so that we can resist the rollout of the 20 mph zones, but to back our campaign to ban bicycles from the streets.

But will he provide it? We asked last week for that data:

---------- Forwarded message ----------
From: Bristol  Traffic <bristol.traffic at>
Date: 13 September 2013 09:17
Subject: Press Query about speed limits in the downs


The Bristol Traffic Project is -as you may know -one of Bristol's premier online reporting sites, focusing purely on transport.

While we are often regarded as some form of shallow spoof, we consider ourselves an evidence-driven organisation: we use photographs and videos to defend our statements, while our coverages of game theory as applied to Bristol's streets include articles that have been cited by such luminaries as Tom Vanderbilt, author of the book, Traffic.

Accordingly, we'd like to follow up our coverage on the proposals for 20 mph limits in the downs, with a query for some data to back up the decision of the committee to retain a 30 mph limit.

One statement you have apparently made to the evening post was that a 20mph limit is inappropriate for the downs: I think it will cause road rage incidents and a lot of frustration for motorists on The Downs."

Here are our questions

1. Do you have any defensible data that demonstrates that 20 mph limits lead to road rage?

2.  London's Royal Parks all have 20 mph limits. What is unique to Bristol's parks, or its residents, that mean that having a 20 mph limit in an urban park is something which they are unable to cope with?

3. Did the downs committee consider how the retention of a 30 mph limit will continue to make cycling on these roads intimidating, especially for families -and that either this would be at odds with the management plan's stated goal of encouraging cycling -and likely to push the cyclists, especially family groups, onto cycling on the footpaths instead?

We await your responses


Chief Data Scientist,
Bristol Traffic Project

---------- Forwarded message ----------

To date we have heard nothing, even though a week has passed. Presumably Cllr Abraham is a busy man, but even so we are concerned that this data -which would be invaluable to the debate about cycling as well as 20 mph limits is being held back.

We shall have to follow this up with another request.

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