They're getting their traffic regulations up to date on the Capitol grounds. Starting this Saturday, streetcar motormen will no longer be subject to citations for violating those rules. Never mind that the city's last streetcars, on the No. 54 and 90 lines, penetrated the Capitol grounds 21 years ago last January.

The motormen's exemption is part of a general updating of the Capitol traffic regulations by the Capitol Police Board, designed to make the general rules identical with those elsewhere in the city. Some, of course, apply specifically to the congressional reservation.

A rundown of the changes scheduled to take effect on Saturday indicates how much things--minor things, so far as most of our lives are concerned--have changed since the Capitol regulations were last amended. For example:

* The old Capitol traffic rules referred at one point to the Commissioner and at another point to the Board of Commissioners of the District of Columbia. The Commissioner is now the Mayor; most duties of the old Board of Commissioners (and some that long belonged to Congress) are now prerogatives of the D.C. City Council.

* The D.C. Department of Vehicles and Traffic, mentioned in the existing Capitol traffic rules, has become the Department of Transportation.

* The D.C. Public Utilities Commission has become the D.C. Public Service Commission (although, in announcing the change, the Capitol Police Board said it is the Public "Services" Commission).

* The D.C. Municipal Court has become the D.C. Superior Court.

And a traffic citation on Capitol Hill is equally valid, whichever rule is in effect. Except for motormen.