Why don't they just say at night around midnight or 3am. At least last year the website gave an hour by hour break out and analysis.
http://www.baybridge.maryland.gov/best-times-to-travel
Thursday Before 10 a.m. and after 10 p.m.
Friday Before 8 a.m. and after 10 p.m
Saturday Before 6 a.m. and after 7 p.m.
Sunday Before 8 a.m. and after 10 p.m.
Monday - 4th of July Before 9 a.m. and after 10 p.m.