Most people view parking meters as a necessary evil, or perhaps not even necessary. Although parking meters manage curb parking and provide public revenue, they are a tough sell to voters. A new type of meter, however, can change the politics of parking by allowing cities to give their own residents discounted prices for curb parking.

In Miami Beach, for example, residents pay only $1 an hour at meters in areas where nonresidents pay $1.75 an hour. Some British cities give the first half-hour free to residents. Cities can also give resident discounts in municipal parking lots and garages. Pay-by-license-plate technology enables these resident discounts.

Drivers pay by cell phone or they enter their license plate number at a multispace parking kiosk and pay by cash or credit card. Linking payment information to license plates lets enforcement officers know which cars have paid for parking. Pay-by-plate meters are common in Europe, and several U.S. cities, including Pittsburgh, now use them. These meters can automatically give discounts for curb parking to all cars with license plates registered in a city.

Like hotel taxes, parking meters with resident discounts can generate substantial local revenue without unduly burdening local voters. This price break for city plates should also please merchants because it will give residents a new incentive to shop in their hometowns. More shopping closer to home might even reduce total vehicle travel in the region.

If resident discounts make parking meters politically acceptable, cities might use further discounts to achieve environmental goals. For example, cities can give discounts for cars that pollute less by linking license plate records to emissions data from car manufacturers or smog tests. Parking meters in Madrid already charge 20 percent less for clean cars and 20 percent more for dirty cars. According to the head of the city's sustainability division, "We thought it would be fair if the cars that pollute more pay more, and compensate those who use more efficient vehicles."

Cities can also classify license plates by car length and give discounts for smaller cars that take up less space at the curb. Because smaller cars tend to be more fuel-efficient, parking discounts for smaller cars will reduce both fuel consumption and carbon dioxide emissions. Parking discounts based on car size will not only provide local economic benefits but also reduce global environmental costs.

If cities want to reduce carbon dioxide emissions, they don't have to wait for state or federal action before they offer parking discounts for smaller cars. Each city can choose its own discounts according to its own priorities.

Parking prices already differ among and within cities, and so can parking discounts. Cities may have to raise their pre-discount meter prices to prevent overcrowding the curb spaces, but only nonresidents with the biggest and dirtiest cars will pay the full pre-discount prices.

Price is the most reliable way for cities to send signals about the behavior they want to encourage, and parking meters can easily send these price signals. If meters give discounts for smaller and cleaner cars, then more people will drive small, clean cars.

Parking discounts may seem complicated, but few drivers will be confused by or object to discounts automatically given at meters. The meters can even print the discounts on parking receipts to reinforce the rewards of shopping close to home and driving small, clean cars. The resident discounts will appeal to local voters and the other discounts will achieve public goals.

Another way cities can increase political support for parking meters is to dedicate the meter revenue to improve public services on the metered streets, such as repairing sidewalks, planting street trees and putting utility wires underground. Some cities offer their neighborhoods a package that includes both parking meters and the added public services financed by the meters. Meters not only manage the curb parking but also provide a steady stream of revenue to pay for public services. People who live, work and own property in the neighborhood can see their meter money at work.

Many cities have scarce curb parking, polluted air, poor public services and political opposition to parking meters. To solve these problems, cities can charge fair market prices for curb parking, give discounts for residents, smaller cars and cleaner cars, and spend the meter revenue to improve public services. Parking meters can then do a world of good.