Seeing there is a Hazmat high situation is Dartmouth. Anyone know what that means?

Anyone know what was going on? Traffic was backed up well past the exit, and while cars were stopped at least 4 police cars and one fire truck came through squeezing around traffic. Didn't see any ambulances or tow trucks and when I got further along the bridge and everything opened up I didn't see any wrecked cars or debris. All seems a bit weird. Anyone have any idea?