The bridge is especially crusty during these times of year... The bridge is from 1936 and has endured this long but barely... This is why they will be tearing it down within the next year and a half.
Freeze and thaw action causes chunks of concrete to fall from the roof sometimes. Be careful while skating down there or even better stay away till the low temps are done for the season. Definitely do not park your car under there if you value it...
More info on freeze and thaw...