* The Issue: Shall an additional sales and use tax of one-half cent per dollar on non-food items be imposed on nine Northern Virginia jurisdictions to fund transit and road projects? The tax is now 4 1/2 cents.

* The Decision: The outcome will be determined by a majority of the total votes cast on Tuesday in the nine jurisdictions affected by the tax.

* Who Spends: The tax money would go to the Northern Virginia Transportation Authority, a new regional agency created by the state. The authority's 14 voting members include the chief elected officers of each county and city in the region.

* Proponents Say: Northern Virginia's traffic congestion problems are among the worst in the nation. The region must begin to make its own decisions about transportation improvements. The tax is important to the entire region's prosperity and will help the environment.

* Opponents Say: The tax is nothing more than a gift to big developers, seeking a new source of government spending to expand the region's road and transit network and open up more areas for housing, office and shopping center construction.

* On the Web: A primer on the tax referendum and other election features are available on The Post's Web site, at www.washingtonpost.com/metro.