The second was opposite a T-junction and had been mostly cleared by the time I got there, but a colleague who turned up earlier reported that the police had earlier closed both roads (which he discovered by trying the first and detouring to the second).
I wonder if it's related to daylight savings? Here the clocks have just gone forward an hour (which is most unfair when it happens the same day as the F1 starts at 7am), and I vaguely remember seeing something about how there's a spike in accidents on the days after daylight savings changes.