It was late and I was falling asleep listening to Bach on Radio 3; so I searched for some enlivening Eminem on Xfm. Suddenly my senses were assailed by the sound of the street: screaming into the middle of this deserted south London road ran a couple. My immediate reaction was to swerve and hurry on home.
But as I drew nearer I realised this was no drunken caper. Blood poured from the woman's head. "Stop!" she screamed. "Let me in." Instinctively I slammed on the brakes and pushed open the door. But the man grabbed her, dragging her into darkness. This had a horrible Bonfire of the Vanities feel, but even a coward like me could hardly speed off now.
I swung the car to the side of the road and ran back. There was no time to think, but I was aware I was pumping with adrenalin - or was it fear? Her attacker had vanished, but she was there, slumped on the curb, blood everywhere, screaming.
Her name is Sarah. She is 24, polite, charming even. She told us she had been stabbed in the hand and bludgeoned over the head with a crowbar and, bizarrely, a Hoover. Another passer-by tried to stem the blood while I phoned 999.
"What's your location?" the police demanded. I didn't know the street name. "Never call the police unless you have an exact reference," the voice admonished me.
"OK, I'll remember that next time I pass a woman with blood pouring from her head," I shouted. So much for technology that maps mobile-phone calls. Eventually, the voice promised an ambulance and squad car. I asked Sarah if there was anyone else I could call. "Mum," she sobbed. So I called her mother.
Talking to her, Sarah broke down once more. She then told this group of strangers her story. "He promised," she cried, "he wouldn't do it after last time." Her attacker, she explained, was her boyfriend. With the blood-flow staunched, she received - I realise now - a deeply unhelpful lecture from me about abusive relationships.
As we waited for the police a man staggered into view. This produced more cries from Sarah: "Keep him away from me," she shouted. Thinking it was the attacker, we moved forwards. But it was the couple's neighbour, who had witnessed the attack.
"Why didn't you do anything?" Sarah demanded.
"I thought he would kill me, too," the man shouted. "I was terrified." Eventually, police and ambulance arrived. I heard on walkie-talkies that the alleged culprit had been arrested: "He has needle marks all up his arm."
We passers-by comforted Sarah, inadequately. The police advised us to go, but I asked Sarah not to see the man again.
"I promise," she said softly. "Thank you for stopping. No one else did."
That's what hit me: no one else did. The assault had been going on long enough for people to take in the scene and speed by, back to their safe lives. Only on the way home did I realise I was lucky that the man ran off; I doubt I'd have done too well in a knife fight. But mostly I thought of Sarah: how had she become embroiled with this man?
Next morning I noticed my passenger window was streaked with blood. I turned on the radio. A debate: 17 youths had been stabbed to death in the capital over a year, yet Boris Johnson had advised Londoners to walk on by.
I was angry. He was urging people like me to ignore the distress of people like Sarah. Surely what we need is more active citizenship, not less. Unless we intervene, however inadequately, our streets will be lost. Judging by how long we waited for the emergency services, they can't be relied upon.
The police have called and asked me to be a witness. "Of course," I said. "For Sarah."