How should the urban cyclist compute personal risk with the laws of the road? This question was brought forcefully home to me the other day when a police car screeched to a halt behind me at a crossroads, siren shrieking, and the policeman inside gestured at me to dismount.

"Were you aware that you just shot a red light?" he barked. One of the cardinal rules my mother taught me was never to answer a policeman back, but it was hard to let this one go. "Well, actually," I blurted out before I could stop myself, "I did stop."

"Yes, but not in the cycle box. Are you aware that it's an offence to proceed beyond the advance stop line when the lights are red?" growled the policeman, and I had to admit that I suspected as much. I grovelled and he drove triumphantly away.

At first I was relieved, but as the adrenaline subsided I began to feel resentful. I am not in the habit of jumping red lights, but I pass this particular crossroads every day, and it is dangerous.

It's a two-lane road that narrows to a single lane on the other side of the junction. The road that crosses it comes in at an angle, so that there is an unusually long stretch of double-yellow line - more than eight metres, compared with the usual four - between the traffic lights and the left-hand turn.

There's a nearside cycle lane leading to a green cycle box, so the instruction seems to be to wait dutifully in the gutter until the lights change - then go for it! Which is fine if you're one of those racers who can go from 0-20mph in 10 seconds, but some of us take longer to get up to speed. Twice I've been barged to a standstill and squashed into the pavement by impatient motorists accelerating past me around the corner.

So my attempts to abide by the law have placed me in one of the highest-risk categories of urban cyclist - a woman negotiating a left-hand junction. Yet I can't do anything about it without risking a hefty fine.

What's to be done? One answer might be to approach the cycle box from the middle of the traffic, keeping to the right of the cars that are going to turn left, but the bottleneck immediately after the junction means that lane discipline is the last thing on motorists' minds - I've also nearly been squashed between two cars trying to race each other to the single lane ahead.

And anyway, when CyclingEngland.co.uk looked into the legal position on advance stop lines (ASLs), they discovered that cyclists were only allowed to enter a cycle box via a designated cycle lane. It's technically illegal to do so by crossing the stop line.

Another possibility would be to ride down the cycle lane to the head of the traffic, then swerve to the right so as to stop in the middle of the box instead of on the nearside - but I hate to think what would happen if the lights changed in mid-manoeuvre.

If I continue stopping ahead of the ASL on the basis that it's the safest thing to do, what legal basis would I have to defend myself? "I suppose you could always twist the truth, and pretend there were loads of cars and motorbikes parked in the ASL, forcing you to move ahead of it," sighs one cycle activist.

Islington Council, whose junction it is, tell me that there is no statutory obligation on the placement of ASLs, only Department of Transport guidelines and good practice. In other words, I can't challenge the layout of the junction itself.

In theory I'm all in favour of ASLs, but theory is no friend to the urban cyclist. The Cyclists' Defence Fund has successfully backed a case involving a cyclist's right to use the road in preference to a separate cycle path, but has no record of legal cases involving cyclists being prosecuted for incorrect use of ASLs.

Ralph Smyth, a barrister who chairs the London Cycling Campaign Policing and Enforcement Working Group, says: "I suspect your experience was in the City of London [actually, it wasn't; it was in Islington], which seems to notch up more enforcement action against cyclists than the rest of the UK combined, at least for red lights.

"There are not many prosecutions for cyclists failing to comply with a stop line: normally police issue a fixed penalty notice (FPN), which you have a couple of weeks to pay or it will go to the magistrates' court for a fine to be issued in default as very few of these sorts of case are contested.

"I have had some unconfirmed reports of police issuing FPNs to cyclists who have stopped beyond the bike box. The City police have confirmed that they would not take any action against cyclists failing to enter an ASL via a cycle lane as they are aware that any such enforcement may increase the number of cyclists who are 'left-hooked' and thereby increase injuries and deaths."

It will no doubt take a spate of accidents for anything to be done to sort this particular junction out. Until then, cyclists just have to make their own risk assessment and face the consequences.