Slow intersection huh. More time for more people on a busier street to hear and see the police car and siren and wonder over. So maybe this commotion and crowd gathering was in large part there for the officer's presence. I know when there's a siren in NY people gather, not a good habit but people do.
There is always a crowd in Harvard Square, period. Its the busiest part of Cambridge. The officer stated he was in his cruiser on Harvard near Ware, its likely that he did not put his siren on.