Most tolls on roadways are flat tolls -- everyone pays the same price, no matter what time it is or how many people are in the car. The point of these tolls is simply to raise money. If the point of a toll is to reduce traffic, you need to take a different approach.
Congestion pricing, sometimes called value pricing, charges drivers based on road demand. The idea is if drivers have to pay more to use a particular stretch of road between, say, 7 a.m. and 8 a.m., they'll only use it if they really have to. And since most drivers on the road during peak times are not actually driving to or from work, getting even a small portion of "discretionary traffic" out of the way can make a big dent in congestion rates -- and, by extension, in greenhouse-gas emissions and commuter travel times [source: FHWA]. (Most tolls are collected electronically, using road sensors and car-based transponders, so drivers don't even have to slow down to pay.)
The overall idea behind congestion pricing is the same across the board, but there are several different approaches to this type of tolling:
Variably Priced Lanes: During peak hours, cars are charged to use fast-moving lanes. San Diego has implemented this strategy in its I-15 HOT lanes. A high-occupancy vehicle can use the HOT lane for free, but a single-person vehicle has to pay a toll to use it.
Variably Priced Tolls: All cars have to pay the toll, but during peak hours the cost of the toll goes up. Two bridges in Fort Myers, Fla., use this approach, dropping the toll price just before and just after peak travel hours, resulting in a 20 percent reduction in traffic during rush hours [source: FHWA].
Zone-based Pricing: Cars have to pay to enter particular areas, typically business-concentrated city centers. Singapore has been using zone-based pricing since 1975.
Variable Area-wide Pricing: Per-mile charges on roadways increase during peak hours. Portland, Ore., which has been experimenting with per-mile road-use charges to replace gas taxes, has tested a variable-pricing system to reduce congestion.
The common thread between all of these methods, aside from trying to get unnecessary vehicles out of the way during peak hours, is that, somehow, they all seem to work pretty well.