Wednesday, 16 September 2015

20mph speech

I rise to speak as a strong supporter of the roll out of 20mph speed limits in Bristol.

As such, I would like to take this opportunity to thank the Liberal Democrats for being the first to agree the roll out of 20mph limits in Bristol, and to thank the mayor for continuing with the rollout.

The roll out of 20mph is only possible because it has gained a significant amount of funding from the LSTF programme - money awarded to Bristol with government approval. This means two things - on e is that those who say ‘why not spend the money on this  or that’ well, the argument simply does not work. We only have the moneyBut it also means that the rollout its only going ahead because the Tories approved the cash in the first place. I given them credit, therefore, for their part in this excellent initiative.

I’d also like to congratulate the organisors of the anti-20mph petition. I have seen the leaflets - this is clearly a well-orchestrated, and well funded campaign. I will leave it to others to say whether the fact that 3,000 of the 8,000 who signed who don’t live in Bristol at all is a good thing or a bad thing. I also congratulate the organisors of the pro-20mph petition - who despite starting much later than anti’s - have gained well over 2300 signatures - it would not surprise me at all if they went on to push this number of the 3,500 limit and therefore force what would be the 4th debate on the subject in about a year.

Looking in more detail at the petitions, I observe that – despite starting a pro-petition four and a half months later,  there are already more supporters of 20mph in the wards of Ashley, Bishopston, Cabot, Cotham, Easton, Redland, Southville and Windmill Hill – all areas with 20mph. When you get it, it seems you like it.

What interest me more, however, is the reasoning that some of those living locally to me give for supporting the anti-20mph petition.

Some just want it gone. However, there are others who have told me either that they support having 20mph to within half a mile or a mile of locals schools. Or they support it in residential areas.

I have to say, if you apply that rationale to the Greater Bedminster area – well, it covers the whole area. I suspect it pretty much covers the whole of the city

The argument – if there is one – is about one or two major streets. And I observe that the 20mph rollout is not ‘blanket’. Many of the major roads in the city remain at 30mph or higher.

One of the arguments against 20mph is that it causes pollution. It isn’t true. There is a study covering central London – by Imperial College – that’s peer reviewed science – which concludes:

It is concluded that it would be incorrect to assume a 20mph speed restriction would be

detrimental to ambient local air quality, as the effects on vehicle emissions are mixed’

It goes on to say

‘At lower speed limits, it is expected that changes in average speed and accelerating and decelerating behaviours will reduce transient demand for power. This in turn is expected to be beneficial to non-tailpipe emissions of particular matter.’

‘It was therefore concluded that air quality is unlikely to be made worse as a result of 20mph speed limits on streets in London’

I would add that this conclusion was borne out by the council’s air quality experts at a meeting of the Place scrutiny commission last week. Many of us are aware that it is down to how you drive – how often you change gear and how much you accelerate or decelerate – which is a major determinant of fuel efficiency and therefore air quality.

The anti-petition mentions retaining 20mph outside school and hospitals. Well, I have been told that upto 80% of accidents to children take place away from schools. Surely, surely – if it is right to protect our children, it is right to protect them everywhere? We therefore surely therefore – go back to extensive 20mph areas.

And of course, I know – having recently left a cycling charity after 8 years – that a major reason people give for not cycling – is the speed of vehicles on the road. Slow down the cars, and you encourage alternative forms of travel.

We also all know that the slower the speed, the better the chance of survival in an accident.

To sum up. to those people who have signed this petition on the basis of changes to one or two major roads, then I say, ok, lets have that debate. Although I believe the case for slower speed is likely to be strong.

To those who want 20mph gone, be clear – even if you can find a mayoral candidate foolish enough to support your cause, be clear, we will fight you all the way.

I sincerely believe that in years to come, we will look back and regard it as anachronistic that anyone doubted the merits of slower speeds in our cities.  20mph is good for the young, good for the old, good for communities and deserves our fullest support.

Thursday, 10 September 2015

Henbury loop analysis (well, moan....)

I have started to get a bit annoyed at the unscientific way in which the case for the Henbury spur has been dealt with, so I thought I'd blog about it, and the lack of science used in calculating the results

The papers recommend:

1) To endorse the Preliminary Business Case and the progressing of Option 1A
(Henbury Spur plus Yate Turn-back), without Constable Road Station, to the
Outline Business Case (Programme Entry).

Essentially, the report uses something called a GRIP2 (or it might have been a GRIP4) analysis of information to come up with a benefit:cost ratio of the Spur v the loop.

I don't intend to go in to the detail but at a basic level:

  • To produce a benefit cost ratio of take up of particular service will involve a large number of factors. Populations, catchment areas, policy decisions, growth rates in train usage, variations in such take up by location etc etc
  • Each factor will have a level of accuracy (or inaccuracy) associated with it 
  • The accumulation of all these factors will result in a large error
  • The analysis is apparently done over a 60 year period. This will exacerbate the problem significantly.

From my memories of doing science (ok, a long time ago....), you would calculate an error to a 95%confidence limit.

This means you are 95% confident that your answer is correct between (Your value minus the error limit) to (Your value plus the error limit). Each value in that range is equally likely.

Hence a BCR of 2 with an error of 1 means the value is somewhere between one and three, you can not say with any more accuracy than that.
The values for the loop are 'BCR with WBS' = 1.35.

If the error margin is 0.7, it gives you a range of 0.65 to 2.05 (which is significant because a 'BCR' of 2 is required to qualify as a scheme.

Likewise, the BCR for the spur with WBS is 2.46. If it had an error margin of 0.7, the range would be 1.76 to 3.16. You'll note that some of these values are lower than some of those above. Given that each value in either range is equally likely, then you simply cannot say that one scheme is better than the other.

The obvious question to ask then - is what error margins/confidence limits did those advocating the Henbury spur come up with?

Answer: they didn't calculate one.