暢通

unimpeded / free-flowing / straight path / unclogged / move without obstruction

free / unimpeded / smooth / at ease / free from worry / fluent

to go through / to know well / (suffix) expert / to connect / to communicate / open / to clear / classifier for letters, telegrams, phone calls etc

classifier for an activity, taken in its entirety (tirade of abuse, stint of music playing, bout of drinking etc)