One and and half car lengths

According to the Highway Code braking distance charts, the "average car" is 4 metres long. The stopping distance available in the allinson crash was 6.65m

Wait for a day when the road isn't wet and slippery

Find a flat, traffic free road,

Find two parked cars in a row. 

Not estate cars, or bloated SUVs, just what's left of what was "an everyday" car at the time the highway codet was written: an Astra, Focus, Civic, etc.

Go up the road from them, 50 m away

Get on your bike: one with a computer, accelerate to 18 mph and stay at that speed, pedalling at a cruising speed. 

As you approach that front car, brake. Hard. Can you stop before your front wheel reaches the end of that second car? Because that is what the guilty verdict in the Allison case turns on. 

Except: that's your stopping distance where you are anticipating the stop, where you know when to brake. Which is exactly what the police video shows they did. 

Now go online to a braking distance calculator: http://www.brakingdistances.com/18Mph

Look at those numbers. 5m "braking distance" -that's in a car, where, as Martin Porter QC covers, can put a lot more braking force in. Your stopping distance in a bike is going to be worse, even with a lot less momentum to burn off.

What's alongside the actual braking distance? Thinking distance. 5 metres. Giving a total stopping distance of 10 metres. Two and a half car lengths.

Similarly, go the official Highway Code stopping chart, look at the closest numbers it has: 20 mph. Six metres for a car to stop, and equally critically, 6 metres of thinking distance.

You cannot correctly estimate the stopping distance in response to something happening in front of you without including that thinking time. Which is precisely why it is included in the Highway Code numbers. 

So why has it been left out here?

Charlie Allinson is the poster child of everything people hate about cyclists: a hate figure the press can vilify, that none of their readers can relate to. And the fuckwit took their front brake off. Which is used as the focal point of the prosecution, and the public condemntation of all cyclists today. 

If he had a front brake, would he have been able to stop in time? 

That is not a question the met office experiment answers, because it doesn't include that thinking time. 

So get out there, find that road, and measure your bikes stopping distance when you are anticipating coming to a halt. Then add five metres of thnking time. Is the total more than 6.65m? Or less? Because, if, our experience holds: there is no fucking way you can do it, even with a mountain bike in full emergency stop mode: disk brakes on hard, fat rear tyre locking up, rider pushing their arse out the back and low to keep that rear wheel weight up. Irrespective of bike, once thinking distance is added in, even that police experiment implies a collision was, sadly inevitable. The kind of crash where in a car or van, the "just came out of nowhere" defence would be wheeled out and the driver would walk out of court, as worse with a few hundred pounds fine from a guilty plea of "death by careless driving"

If a collision seems inevitable the, what did the lack of a front brake do?

it may have meant the rider was going faster when he hit Kim Briggs. A slower speed collision may have meant that her family wouldn't have got the worst news of their lives. It may also have given him some control in the crash: possibly even enough to steer around her 

But there is no way, front wheel or not, someone on a bike or car travelling at 18 mph can come to a unanticipated stop from in 6.65m. The sole defence there is for everyone, car or bike, to recognise when they don't have visibility, slow down and and be ready for someone stepping out. 

Which is something we all need to do: otherwise, it's us next in court, getting crucified by the press "they mowed her down". Or the far more frequent and equally tragic case, walking out of court with the "they came out of nowhere" defence working, nothing but a small mention in the local press -and a family left to mourn about the injustice of a road safety system where they can prosecute a cyclist for not stopping in under 7 metres, but let speeding drivers off with an apology "it wasn't your fault"