How Traffic Works

Traffic Control

Boston's "Big Dig," a $14.6-billion project, sought to alleviate traffic by creating a 3.5-mile highway under the central artery of the city.
Boston's "Big Dig," a $14.6-billion project, sought to alleviate traffic by creating a 3.5-mile highway under the central artery of the city.
Photo John Mottern/Getty Images

Most reports on congestion focus on highways, but surface streets can have their own share of problems, particularly as suburban populations increase.

Civil engineers have to take many factors into consideration when designing for surface streets. For instance, a poorly designed intersection can be inconvenient or unsafe. Consider all the different elements a civil engineer must tend to: a driver's line of sight, the impact the intersection will have on the streets surrounding it, the amount of traffic the intersection is likely to see and other issues.

Another challenge is suburban sprawl -- neighborhoods that were once less populated might experience a growth spurt accompanied by new needs in road design. ­

­Most cities have a well-established system of roads, making extensive changes impractical or even impossible. It's easy to make suggestions to solve a city's traffic problems, but implementing solutions can be prohibitively expensive. Perhaps the easiest way to make an impact on city traffic is through traffic lights.

Traffic lights typically are on a timed system, a sensor system or a combination of the two. Timed systems follow a set schedule no matter what traffic conditions are like (though the schedule itself might change throughout the day). Sensor systems detect cars as they pull up to the intersection, which triggers a change in the traffic light. Advanced traffic systems network signals to a master computer system. A good system uses signals that are timed together so that the flow of traffic remains as constant as possible. However, even a well-designed traffic coordination system will only reduce traffic delay by approximately 1 percent [source:TTI].

Another way to control traffic dispersal within the city is to institute turn prohibitions and auto-restricted zones. Turn prohibitions mean you can't turn at specific intersections or points on a road, which channels traffic into alternate routes. Auto-restricted zones are areas where cars aren't allowed at all, usually to facilitate pedestrian traffic or preserve a historic district in a city or town. In Boston, for example, you can find the Downtown Crossing Project, an auto-restricted zone encompassing 12 city blocks [source: TTI].

Traffic experts like Alistair Darling, the British Secretary of State for Trade and Industry, suggest that the most effective means of reducing congestion -- both on highways and surface streets -- is to institute congestion pricing. His philosophy is that drivers exact a cost on a road (through wear-and-tear and environmental impact), and that they should pay a price to offset the cost. In other words, you would have to pay to drive on city streets. It's similar to the concept of toll roads, but is a little more complicated.

A true congestion-pricing system would track each driver as he maneuvered around city streets using an electronic system of sensors. Each car would have an electronic identifier unique to the vehicle, similar to a radio frequency identification tag. Rates might vary throughout the day, generally reaching the highest point around rush hour. Driving on city streets during this time results in a fine. Because no congestion-pricing systems currently exist, there are no specific rates or fine-collection techniques to speak of at this time. Critics of congestion-pricing systems point out that such a system would likely be a political impossibility because drivers have become used to driving on city streets for free. A similar system in Seoul, Korea faced massive public opposition, including charges that the city was imposing a tax on drivers [source: IGES].

In the next section, we'll look at ways you can help reduce traffic congestion.