There is one place in Washington that is exempt from the city's tough new drunk driving law -- but it won't remain so for long.

The place is the United States Capitol Grounds, an enclave where traffic rules are not drawn by the D.C. City Council or even by Congress itself, but by a three-member body of appointed congressional staff officials called the Capitol Police Board.

The board, saying it recognized "the importance of promoting uniformity" of traffic laws and "the substantive merits" of the Council's new drunk driving law, has voted to adopt identical rules covering its Capitol Hill enclave.

Tickets issued on Capitol Hill, whether by the Capitol or D.C. police forces, are tried in the same court, D.C. Superior.

But until the new Capitol Grounds rules go into effect on Oct. 9, the end of a statutory waiting period, an arrest for a second or subsequent drunk driving offense will be treated less severely than elsewhere in the city. However, the new city law, which went into effect last Saturday, reduces the maximum penalty for a first offense.

Under the city law and the new Capitol rules, persons arrested with blood alcohol levels of 0.1 percent may be convicted of driving while intoxicated even if that driving is not erratic.

At least three major commuter traffic arterials pass through the Capitol Grounds -- Constitution and Independence avenues and Canal Street SW. Other streets affected include portions of Delaware and New Jersey avenues and First, C and D streets NE-SE.

Benjamin J. Guthrie, sergeant at arms of the House of Representatives, is chairman of the Capitol Police Board. Its other members are the sergeant at arms of the Senate and the architect of the Capitol.