Archive for biking

wanton and furious cycling

Posted in pictures, Running, Travel with tags , , , , , , , , on September 10, 2017 by xi'an

A cyclist was convicted of “wanton or furious driving” last week in London after hitting a pedestrian crossing the street, leading to her death a few days later. The main legal argument for the conviction was that the cyclist was riding a “fixie”,  a bike with no front brake and fixed-gear, as used in track cycling. Which is illegal in Britain and, I just found out, in France too. (He was actually facing manslaughter, for which he got acquitted.) This is a most tragic accident, alas leading to a loss of a human life, and I did not look at the specifics, but I do not get the argument about the brakes and the furious driving: if the rider was going at about 28 km/h, which seems a reasonable speed in low density areas [and is just above my average speed in suburban Paris], and if the pedestrian stepped in his path six meters ahead, he had less than a second to react. Front brake or not, I am certainly unable to react and stop in this interval. And braking hard with the front brake will invariably lead to going over the bars: happens to me every time I have to stop for a car with my road bike. And would if I had to stop for a pedestrian.

Incidentally [or accidentally], here is the item of British Law from 1861 on which prosecution was based:

“Whosoever, having the charge of any carriage or vehicle, shall by wanton or furious driving or racing, or other wilful misconduct, or by wilful neglect, do or cause to be done any bodily harm to any person whatsoever, shall be guilty of a misdemeanour, and being convicted thereof shall be liable, at the discretion of the court, to be imprisoned for any term not exceeding two years.”

And here are the most reasonable views of the former Olympian Chris Boardman on this affair and the hysteria it created…

lesson, test, and results

Posted in Running, Travel with tags , , , , , , on June 7, 2017 by xi'an

Two weeks ago, I got stopped by the traffic police in Paris for crossing a red light with my bike. Or more exactly two red lights in a row (the third one having turned green seconds before I went through). The surprise is that it only happened then, given my almost daily ride to Dauphine and my illegal if cautious management of the numerous red lights on my usual route. The policeman was quite polite, given the obvious break of the law, and asked me if he should fine me for the two red lights, at €135 each! Question to which I replied that he was The Law and I could not argue the facts. In the end, he gave me a lecture on the (real) dangers of crossing red lights—which in my opinion and experience are lesser than those of parked cars and scooters pulling out or opening doors without checking first—and warned me that I would get a fine if our paths crossed again at a red light. Which is just fair (even though I should have gotten the fines, in all fairness).

On the way back that evening I decided to count the exact number of lights and the extra-time it would take me when stopping at all red lights. Surprisingly, it only added six minutes to the 30mn± trip (not accounting for the variability on other days, my best time ever being 26:59 two weeks ago, on ±13km), despite the 60-ish lights on my way home. The experiment did not convince me to keep stopping at all red lights, since I find restarting from static always a major pain, but I now pay more attention to my surroundings when doing so. Until I find the technique to run by foot through the lights (which is legit!)…

crossing the Seine

Posted in pictures, Running, Travel with tags , , , , , , , on December 16, 2016 by xi'an

À l’Observatoire de Paris

Posted in Kids, Mountains, pictures, Running, Statistics, Travel, University life with tags , , , , , , , , , , , on July 5, 2016 by xi'an

This Monday, I made a most pleasant trip to the Observatoire de Paris, which campus is located in Meudon and no longer in Paris. (There also is an Observatoire de Paris campus in downtown Paris, created in 1667, where no observation can take place.) Most pleasant for many reasons. First, I was to meet with Frédéric Arenou and two visiting astrostatisticians from Kolkata, India, whom I met in Bangalore two years ago. Working on a neat if no simple issue of inverted mean estimation. Second, because the place is beautiful, with great views of Paris (since the Observatoire is on a ridge), and with a classical-looking building actually made of recycled castle parts after the Franco-Prussian war of 1870, and because Frédéric gave us a grand tour of place. And third, because I went there by bike through the Forêt de Meudon which I did not suspect was that close to home and which I crossed on downhill muddy trails that made me feel far away from Paris! And giving me the opportunity to test the mettle of a new mountain bike elsewhere than again Parisian SUVs. (This was the first day of a relatively intense biking week, which really helped with the half-marathon training: San Francisco ½ is in less than a month!!! And I am in wave 2!)

Robert’s paradox [reading in Reading]

Posted in Statistics, Travel, University life with tags , , , , , , , , , , , , on January 28, 2015 by xi'an

paradoxOn Wednesday afternoon, Richard Everitt and Dennis Prangle organised an RSS workshop in Reading on Bayesian Computation. And invited me to give a talk there, along with John Hemmings, Christophe Andrieu, Marcelo Pereyra, and themselves. Given the proximity between Oxford and Reading, this felt like a neighbourly visit, especially when I realised I could take my bike on the train! John Hemmings gave a presentation on synthetic models for climate change and their evaluation, which could have some connection with Tony O’Hagan’s recent talk in Warwick, Dennis told us about “the lazier ABC” version in connection with his “lazy ABC” paper, [from my very personal view] Marcelo expanded on the Moreau-Yoshida expansion he had presented in Bristol about six months ago, with the notion that using a Gaussian tail regularisation of a super-Gaussian target in a Langevin algorithm could produce better convergence guarantees than the competition, including Hamiltonian Monte Carlo, Luke Kelly spoke about an extension of phylogenetic trees using a notion of lateral transfer, and Richard introduced a notion of biased approximation to Metropolis-Hasting acceptance ratios, notion that I found quite attractive if not completely formalised, as there should be a Monte Carlo equivalent to the improvement brought by biased Bayes estimators over unbiased classical counterparts. (Repeating a remark by Persi Diaconis made more than 20 years ago.) Christophe Andrieu also exposed some recent developments of his on exact approximations à la Andrieu and Roberts (2009).

Since those developments are not yet finalised into an archived document, I will not delve into the details, but I found the results quite impressive and worth exploring, so I am looking forward to the incoming publication. One aspect of the talk which I can comment on is related to the exchange algorithm of Murray et al. (2006). Let me recall that this algorithm handles double intractable problems (i.e., likelihoods with intractable normalising constants like the Ising model), by introducing auxiliary variables with the same distribution as the data given the new value of the parameter and computing an augmented acceptance ratio which expectation is the targeted acceptance ratio and which conveniently removes the unknown normalising constants. This auxiliary scheme produces a random acceptance ratio and hence differs from the exact-approximation MCMC approach, which target directly the intractable likelihood. It somewhat replaces the unknown constant with the density taken at a plausible realisation, hence providing a proper scale. At least for the new value. I wonder if a comparison has been conducted between both versions, the naïve intuition being that the ratio of estimates should be more variable than the estimate of the ratio. More generally, it seemed to me [during the introductory part of Christophe’s talk] that those different methods always faced a harmonic mean danger when being phrased as expectations of ratios, since those ratios were not necessarily squared integrable. And not necessarily bounded. Hence my rather gratuitous suggestion of using other tools than the expectation, like maybe a median, thus circling back to the biased estimators of Richard. (And later cycling back, unscathed, to Reading station!)

On top of the six talks in the afternoon, there was a small poster session during the tea break, where I met Garth Holloway, working in agricultural economics, who happened to be a (unsuspected) fan of mine!, to the point of entitling his poster “Robert’s paradox”!!! The problem covered by this undeserved denomination connected to the bias in Chib’s approximation of the evidence in mixture estimation, a phenomenon that I related to the exchangeability of the component parameters in an earlier paper or set of slides. So “my” paradox is essentially label (un)switching and its consequences. For which I cannot claim any fame! Still, I am looking forward the completed version of this poster to discuss Garth’s solution, but we had a beer together after the talks, drinking to the health of our mutual friend John Deely.

a day of travel

Posted in pictures, Running, Travel with tags , , , , , , on September 1, 2014 by xi'an

Bham2I had quite a special day today as I travelled through Birmingham, made a twenty minutes stop in Coventry to drop my bag in my office, went down to London to collect a most kindly loaned city-bike and took the train back to Coventry with the said bike… On my way from Bristol to Warwick, I decided to spend the night in downtown Birmingham as it was both easier and cheaper than to find accommodation on Warwick campus. However, while the studio I rented was well-designed and brand-new, my next door neighbours were not so well-designed in that I could hear them and the TV through the wall, despite top-quality ear-plugs! After a request of mine, they took the TV off but kept to the same decibel level for their uninteresting exchanges. In the morning I tried to go running in the centre of Birmingham but, as I could not find the canals, I quickly got bored and gave up. As Mark had proposed to lend me a city bike for my commuting in [and not to] Warwick, I then decided to take the opportunity of a free Sunday to travel down to London to pick the bike, change the pedals in a nearby shop, add an anti-theft device, and head back to Coventry. Which gave me the opportunity to bike in London by Abbey Road, Regent Park, and Hampstead, before [easily] boarding a fast train back to Coventry and biking up to the University of Warwick campus. (Sadly to discover that all convenience stores had closed by then… )

art brut

Posted in pictures, Running, Travel with tags , , , , , , on July 26, 2014 by xi'an