Voters in this fast-growing Sun Belt community decide Tuesday whether to boost their taxes by $5.8 billion over the next 20 years so they can build a network of freeways once scorned by this area.

Twenty years ago, when the federal government was pouring billions of dollars into the Interstate highway system, Phoenix frowned on freeways. The fear was that Phoenix would become another Los Angeles, with ribbons of concrete stretching in all directions and an abundance of smog.

Now, the Interstate system is virtually complete, and federal contributions are down to a relative trickle.

In the meantime, air quality in the Phoenix area has been getting worse, and traffic on the city's streets creeps. Backers of the new transportation plan say freeways would move traffic more efficiently, thereby reducing pollution. Most of these same freeways have been on drawing boards for at least 20 years.

The issue in Tuesday's election in Maricopa County is whether to increase the 5-cent state sales tax by one-half cent. An additional 1-cent is tacked on by most cities in the area, including Phoenix, so if voters say yes, the sales tax in many parts of the county will be 6 1/2 cents on the dollar.

That would raise an estimated $5.8 billion during the next 20 years, enough to build 233 miles of freeways. Phoenix now has about 70 miles of limited access highways, by far the fewest for a city of its size. By comparison, San Diego has 282 miles of freeways.

Supporters of the transportation plan include key politicians in both major parties and the Phoenix Metropolitan Chamber of Commerce. Nearly $1 million has been contributed to promote, on television and radio, the push for freeways. Most of the money has come from banks and utilities. They warn that traffic jams will only get worse without freeways, and that new businesses are less likely to move to the Valley of the Sun if major steps are not taken to improve transportation.

Opponents, primarily the Arizona Libertarian Party and a taxpayers' group from the retirement community of Sun City, insist that gasoline taxes should be increased to cover the cost of more freeways. Transportation planners have estimated that it would take an increase of 33 cents a gallon to equal a one-half-cent boost in the sales tax.

Legislation providing for the vote in Maricopa County makes similar sales tax elections optional in the state's 14 other counties. So far, only one other county has scheduled an election, and several more have shown an interest.