In DC, the WMATA Metro system seems to be pretty reliable and safe. My D took it during rush hour times from suburbia to downtown DC when she interned starting at age 16 and we never felt cause for alarm. GEN Alum uses it for the Pentagon commute.
In addition to Metro, VRE, Maryland rail, there are HOV lanes and the resultant slugging behavior.
http://www.slug-lines.com/