In short they are  beneficial wasp, Sphecius speciosus or better knon as  Eastern Cicada Killer. Cicada Killer wasps are large, solitary wasps in the family Crabronidae .They are so named because they hunt cicadas and provision their nests with them. Although cicada killers are large, female cicada killer wasps are not aggressive and rarely sting unless they are grasped roughly, stepped upon with bare feet, or caught in clothing, etc.

 They have been observed at the UC balcony  and along Center Rd and Poplar intersection .

Here is a great link with photos and video. http://raupplab.umd.edu/raupplab_java/bow-reader.jsp?/wt/raupplab/bugweek/BugOfWeek_30E.html