Here are the laws for Interstate travel.
http://www.fmcsa.dot.gov/registration-licensing/cdl/cdl.htmWA has adopted the federal requirements (some state have some variations, but it only applies to their residents during intrastate travel). WA has a nice flow chart published:
http://www.dol.wa.gov/driverslicense/cdlrequired.htmlNote: Many commercial insurance policies require a CDL even for combinations that would not otherwise require a CDL by DOT. Your private insurance will not cover you while you are driving for business. Also be careful because some insurers may extend this exclusion to "bobtailing" because you would the empty trip is necessary for completing the haul-job. The "on the ball" coverage provided by the transport company only covers you only while hooked-up (they may not even include liability, only loss coverage for the load), so you may have a coverage gap without commercial insurance.
