Insane claim of the year - the BMJ

Apparently, increasing the speed limit on UK motorways from 70mph to 80mph will increase obesity according to an article in the BMJ:

The authors say a higher limit could increase gas emissions and air pollution and could lead to increased obesity, with more people taking advantage of shorter car journeys.

An exercise for the reader: assuming the driver takes 1 minute from leaving home to joining the motorway at an average speed of 20mph, and 1 minute from leaving the motorway to arriving at destination at the same 20mph average, what is the minimum distance from start to finish for which she would save 1 minute of travel time by travelling at 80mph rather than 70mph on the motorway, assuming instant acceleration from 20 to 80?

Answer: (highlight to see) at 70mph she travels 1.16 miles per minute, at 80mph 1.33 miles per minute, so X/1.16 = 1 + X/1.33, solve for X and you get X(1.33 - 1.16) = 1.16*1.33: X = 1.54/0.17 = 9 miles (plus 2/3 of a mile either side)

I'm glad that the authors of this paper are devoting so much effort to safeguarding those among us who would rather drive 10 miles than walk it because it's one minute quicker than previously.

[I wish the Grauniad had a link to the paper because I'd like to identify the authors and beat the living daylights out of them. For spouting such specious crap, they don't deserve to live, let alone get funding.]