The principle behind it is as follows. Cyclists are enabled to get to the front of a queue of traffic at a light-controlled junction, and wait in a box in front of the queue, where you're visible to motorists, instead of getting in the blind spots of HGVs, which, as we all know, isn't such a terribly good idea. Simple eh? Not to the UK justice system, which conspires to make it both complex and crap.
It wasn't so bad in the old days when the police didn't bother enforcing them. If there were vehicles in the advance stop box, which there usually were, you would just go ahead over the stop line and wait in front of them. Not that big a deal. Against the letter of the law, yes, but the whole point of ASLs is to enable you to be seen and get a head start rather than be mixed up in HGV blind spots. Your personal safety is a greater moral imperative than a technical breach of the rules, especially bearing in mind that you're forced into this minor infringement (usually) by the motorist breaking the law in the first place.
Unfortunately, such moral considerations seem to butter no parsnips with the Met Police, who ticketed a London cyclist for just such an offence. Unsurprisingly, fellow riders have rushed to contribute to a defence fund to mount a legal challenge.
When you drive a car, you will almost always stay safe if you stick to the letter of the law. That's because the law, and the roads, are designed with you in mind. On a bike, the situation is rather different. You continually have to make potentially life-or-death decisions that balance your safety and your speed against staying legal. You have to think about road positioning, your visibility to other road users, parked vehicles, and of course what other moving road users are doing, or might do, or what might happen if they don't do what they appear to be intending. You have to think ahead and analyze every possible move of every road user in a complex, continuous game of chess. You have to look at every approaching vehicle and try to figure out if they are about to overtake you or turn across you, and whether you are better off taking an assertive position to discourage a dangerous manoeuvre, or just let them get on with it. You also have to consider whether they are actually aware of you, or texting their mate instead.
Now, you might say that breaking the law to expedite your journey is not acceptable, but the fact is that being stationary or slowing down can often put you in considerable danger of a rear hit or a left or right hook. Loss of momentum can be fatal. So cutting a corner or going the wrong side of a bollard can be a better option than waiting in the middle of the road, trusting that drivers in front and behind are paying attention, driving cautiously and aware of the fact that there might be a cyclist in front that they can't yet see.
On this blog, we don't condone stupid cycling that endangers other road users, but we also don't condone stupid laws that endanger cyclists, and the fact is the ASL is governed by an embarrassingly badly-drafted, Friday afternoon-after-a-liquid-lunch piece of legislation. Logic would tell you that the advance stop box should behave like a yellow box junction, but it doesn't: it's legal for a vehicle to wait in the ASL if it crossed the first stop-line when the light was green, which makes it very difficult to enforce. There's also considerable confusion about when and how a cyclist can enter the box. According to the CTC:
"the legal position of ASLs has not been clear in the past, either through legislation or case law. So far, ASL layouts have required an approach cycle lane, long enough for cyclists to bypass the traffic queue (though what that meant was not entirely clear). When the lights are red, cyclists may only enter the reservoir via the cycle lane, not by crossing the stop line.
This legislation raised a question over the legality of not having a cycle lane - without one, it has been technically illegal for a cyclist to enter the ASL box in the absence of a green light. Several authorities sought to overcome this by leaving a 1.2m break at the nearside end of the main stop line, possibly with a token length of cycle lane, or tapered feeder lane. The legality of this approach, however, has been unclear (see also 3.c. ‘Omitting feeder lanes’ below).
A question also arose over whether the regulations were too prescriptive on how cyclists are expected to use ASLs with, say, a single feeder lane but two traffic lanes. For example, if a central feeder lane is provided between two lanes of traffic, the legislation requires left turning cyclists to use that lane to access the ASL, even if there is space for them to filter past traffic in the nearside lane, with obvious dangers if the lights change before they get there. "
It seems there should be a fix for this in the upcoming revision of "Traffic Signs and General Direction Regulations" in 2014, but that almost certainly won't fix the problem of what a cyclist is supposed to do if the advance stop box is occupied.
The fundamental problem is with the whole Highway Code, which lumps cyclists in with all other traffic and subjects them mostly to the same laws and regulations. Yet most of the highway code is advisory, so many of the most dangerous driving behaviours are committed on a daily basis with no fear of prosecution. As a result, dangerous driving has become the accepted norm for many: A TRL survey of drivers convicted of careless driving revealed "57% claimed they were driving how they often or normally drove at the time of the incident, and 75% said they were surprised to be convicted". Furthermore, we have a legal system in which it is very difficult to get a meaningful punishment or even a successful prosecution for dangerous driving (drivers can plead 'momentary inattention'/pressed the wrong pedal/uncharacteristic error/didn't-see-you, etc. etc.), yet very easy to prosecute a cyclist who acts to preserve his/her own safety, endangering nobody else, but happens to contravene the letter of a car-centric law.