President Clinton today signed a bill lifting the 21-year-old national speed limit despite fears that the measure will result in an increase in highway fatalities. White House spokesman Mike McCurry said Clinton felt a veto could cost states $6 billion because the provision is tacked onto this year's major highway spending bill. Another reason: the popular bill, which passed the House without opposition and the Senate on a 80-16 vote, is likely to withstand a veto anyway. In a few weeks, reports Austin bureau chief S.C. Gwynne, workers will begin posting signs raising the state speed limit to 70 mph over 77,000 miles of Texas highways. A number of like-minded Western states are giving motorists freer rein: Kansas, Nevada and Wyoming will raise the limit to 75 mph, while Oklahoma and South Dakota will set theirs at 70. The best state for Unser wannabes? Montana, where the only limit will be how fast eight cylinders can move you.