President Reagan signed legislation yesterday to release $7.2 billion in interstate highway construction funds for projects in 42 states.

The measure, sent to Reagan by the Senate on March 5, was adopted under heavy pressure from governors after being held up for 18 months by congressional wrangling.

"States can now move forward with projects to close the remaining gaps in the interstate system, and our goal of completing the interstate system by the early 1990s is once again attainable," Reagan said in a written statement. In a reference to the lawmakers' adoption of a sharply pared, two-page resolution freeing the construction funds, Reagan said he was "pleased with the courage they showed in laying aside numerous provisions of considerable importance to them in order not to further delay release of these funds."

The Senate scrapped its own 34-page version of the measure, which contained an assortment of provisions for distributing the money, and passed a bare-bones House bill.

The administration had pressed for an austere version of the so-called interstate cost estimate (ICE), Capitol Hill approval of which was needed before the funds could be released. The measure was stalled for 1 1/2 years amid House and Senate haggling over home-state "demonstration projects" highly favored by some lawmakers.