How Traffic Works

Many factors can contribute to traffic congestion, but the most basic explanation is that the number of drivers trying to use the same road is so high that it goes beyond the road's capacity to handle cars.
2008 HowStuffWorks

Have you ever wondered how many hours you've spent sitting in bumper-to-bumper traffi­c? According to the Texas Transportation Institute (TTI) at Texas A&M University, you might spend as much as two weeks in your car each year [source: Reason Foundation].

The 2007 study revealed that in 28 urban areas, including cities like Boston, Detroit, Atlanta, San Francisco, Orlando and Minneapolis-St. Paul, drivers spent an entire work week sitting in traffic each year.­ In Los Angeles, the country's benchmark for traffic congestion, it could be up to nearly two weeks.


Traffic has serious consequences, and not just on your well-being. The estimated cost of traffic for 2005 was more than $78 billion (in fuel and wasted time), and that doesn't take into account factors like environmental damage or health co­sts due to pollution [source: TTI]. In fact, Americans bought 2.9 billion extra gallons of gas because of traffic congestion. The average annual cost to an individual driver was $710 [source: TTI].

Many factors can contribute to traffic congestion, but the most basic explanation is that the number of drivers trying to use the same road is so high that it goes beyond the road's capacity to handle cars. That's a pretty simple explanation -- too many cars in ­one place causes traffic. Unfortunately, the underlying reasons for too many cars in one place at one time are more complicated. University departments and civil engineers dedicate hundreds of hours and require millions of dollars in funding to understand how traffic congestion forms and what can be done about it.

City planners, civil engineers, environmental advocacy groups, homeowner associations, politicians and the general population can have a significant impact on how we address traffic congestion. Traffic is a very political and sensitive issue since almost every proposed method of addressing it carries a hefty price tag, raising the question of who pays the bill.

In this article, we'll learn about traffic congestion on highways and surface streets and the options city and state officials have when approaching traffic management. We'll look at ways you can help prevent traffic snarls through your own driving and car-maintenance habits. And in the last section, we'll find out which cities are known for the worst traffic.

In the next section, we'll take a closer look at highway congestion.


Traffic Causes

You can't understand it -- how can a five-lane highway become so packed? It feels as if every car in the city has joined you on the highway at the same time. Traffic creeps forward at a snail's pace, when it moves at all. You're forced to waste time, gas and money. What causes this?

As the first car stops, the following cars must also stop. Even when the first car begins to move again, additional approaching cars have to stop farther down the road, and the congested area travels backward in a wave until traffic is light enough for it to dissipate.

Assuming construction, accidents, and stalled vehicles aren't to blame, it's likely due to more cars entering the highway than leaving it. As more cars enter a crowded road, drivers have to use their brakes to avoid collisions, creating a traffic wave. A traffic wave occurs when cars slow down, and the slowing trend continues backward -- like a domino effect. As long as there are more cars approaching from behind, the traffic congestion travels in a wave.


In general, you can divide up the contributors of traffic into two broad areas: network overload and traffic disturbances.

Network overload

If there are highways or surface streets that suffer from heavy traffic congestion, no matter what the actual road conditions might be, they fall into the category of network overload. These are the bottlenecks and traffic snarls where demand always outweighs capacity.

As space opens up ahead of your car, you can accelerate and escape the congestion. The person behind you can accelerate a few moments later, and the person behind them a few moments after that. The congestion doesn't immediately clear up -- it continues to shift slowly back down the highway. Congestion can clear if traffic becomes light enough to stop the traffic-wave effect.

Traffic Disturbances

Accidents and breakdowns, road construction and repair, and harsh weather conditions are all considered traffic disturbances. You can't always predict where these disturbances will occur, but they still heavily impact traffic flow.

It's easy to imagine construction, an accident, or a cop giving a traffic ticket causing congestion -- drivers slow down either to change lanes or engage in a bit of rubbernecking as they try to see what happened. Road work might shut down one or more lanes, requiring drivers to shift over into open but crowded lanes. Bad weather might cause some drivers to maintain a slower driving speed out of concern for safety. According to the 2007 Urban Mobility Report from the Texas Transportation Institute, traffic incidents account for between 52 and 58 percent of the delays motorists experience [source: TTI].

In the next section, we'll learn about cities and highway traffic.


Traffic Solutions

Travelers in Maryland are warned of changes in traffic patterns as they travel on a seven-mile, two-hour traffic jam caused by construction of a new bridge.
Photo Jim Watson/AFP/Getty Images

Many cities, like Los Angeles, have sophisticated traffic communications systems that alert drivers to changing conditions on the road, giving them time to decide what to do. Several cities have invested millions of dollars so that road crews can rapidly travel to trouble spots.

There are a few ways cities can address highway congestion:


  • Ramp metering - cars are only allowed to enter the highway at timed intervals. This is done by placing a light similar to a traffic signal at the end of the ramp. Cities using ramp metering report a travel-delay reduction of 29.4 million hours annually with a significant decrease in traffic accidents -- yet capacity on the highway actually increased when ramp metering was introduced [source: TTI]. In Minnesota, the DOT instituted a strict ramp-metering program that included 430 ramp meters. As an experiment, the DOT shut off the ramp meters for seven weeks in 2000. During that time, traffic accidents increased by 26 percent. After the experiment, the DOT reinstituted the ramp meters and saw highway capacity increase by 14 percent [source: TTI]. Though ramp metering can increase highway speeds while decreasing accidents, it usually takes a long time to implement and requires thorough investigation to make sure surface street traffic isn't affected as cars back up to enter the highway [source: AGC of America].
  • High-occupancy vehicle lanes (HOV) - many cities have included these lanes on highways. HOV lanes are reserved for cars with a certain number of passengers (usually two or three people per car). Drivers have an incentive to carpool, reducing the total number of cars on the highway. Some HOV lanes have exclusive off-ramps, reducing the need to merge with other traffic.
  • Adding lanes - a common approach to congestion problems is to add lanes to the highway, either by widening the road, decreasing the width of existing lanes or converting a shoulder or other space into a lane. These sort of adjustments are expensive, time consuming and controversial. Several studies suggest that increasing the width of a road only increases the volume of cars without addressing congestion. Other studies say that in many circumstances, widening the road can greatly reduce congestion. According to TTI's extensive research, adding lanes and widening roads only works if capacity stays ahead of population growth [source: TTI].

In the next section, we'll look at congestion on city streets. ­­


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.
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.


Traffic-jam Prevention

In the virtual slot model of traffic, cars inhabit a space around them that other cars shouldn't enter. By maintaining the distance between other cars, drivers should experience smoother traffic flow with fewer congested area.

If you want to reduce your contribution to traffic jams, the first thing you should do is take care of your car. Making sure your car is properly maintained can help prevent breakdowns on the road. Maintenance includes regular oil changes, tune ups and tire care. Make sure your tires are kept at the right pressure -- it's safer and can improve your gas mileage by up to 3.3 percent [source:]. Keeping your car in good working order will save you time and money and might help keep you out of dangerous situations.

When on the road, try to maintain a safe and steady distance between you and the driver ahead of you. Suddenly speeding up -- only to slow down again -- causes drivers behind you to do the same thing, eventually resulting in a congestion wave (and road rage!).


In an article titled "Vision of Congestion-Free Road Traffic and Cooperating Objects," Ricardo Morla proposes we think of cars occupying virtual slots. Each virtual slot takes up a real space that travels at a specific, continuous speed down the road. As cars approach one another, drivers must adjust the speed of their cars so that the virtual slots don't overlap. Morla admits that this system fails whenever more cars enter a highway than the virtual slots can accommodate. Still, by keeping a safe distance between you and other vehicles, you can help minimize congestion [source: Morla].

Driving at non-peak rush-hour times is another good way to avoid contributing to the congestion problem. If you have flexibility with your schedule, you can travel at non-peak hours. Proponents of the congestion-pricing system say that levying fees on drivers during peak hours would encourage people to drive at off-peak times. Critics point out that this comes close to regressive taxation, meaning that the poor shoulder most of the cost. They say that people with flexible schedules tend to be professionals working in white-collar jobs, whereas people who work in lower-paying positions tend to have set hours and are unable to avoid traffic. The people least able to afford the fee would be the ones footing the bill [source: Arnott].

Carpooling is a great option if you live near people who work near you. Most cities have HOV lanes you can use, and carpooling creates­ less stress on the environment, leading to less pollution. Many people are reluctant to give up the freedom they have when driving their own car. Carpooling means aligning your schedule with other people and scheduling any errands or side trips after you get back home.

If your city has a good public transportation system, you can always use it to reduce your impact on congestion. But just like carpooling, using public transportation means giving up some of your freedom and flexibility.

In the next section, find out what cities have the worst traffic.


Worst Cities for Traffic

Cairo, Egypt is one of the world's worst cities for traffic congestion.
Photo Khaled Desouki, AFP, Getty Images

It probably doesn't surprise anyone that Los Angeles tops the Texas Transportation Institute's (TTI) list for the worst traffic in the United States. Films, television shows and songs have all poked fun at the City of Angels' traffic issues. And despite what you might have learned from the television show "24," you can't get anywhere in Los Angeles from anywhere else in the span of 15 minutes. In fact, Los Angeles' travel time index is 1.92, meaning you should plan for a trip during peak hours to take nearly twice as long as it would at an off-peak time of day [source: TTI].

According to the 2000 Census, nearly 81 percent of all commuting workers travel to work in a car, truck or van. Of that group, nearly 66 percent drove by themselves -- only 14.7 percent carpooled. The total number of workers was 1,494,895. Most of those driving traveled during peak hours. Los Angeles leads the nation in time wasted by sitting in traffic -- the average Los Angeles motorist spends 72 hours every year in traffic jams [source: TTI]. That's nearly two full work weeks spent staring at the car in front of you and fighting off road rage.


The other cities rounding out the top five on TTI's list include:

California has five of the top 12 areas for the worst traffic congestion. Most experts predict congestion will continue to increase as populations grow. Some cities you might expect on the list, like Boston and New York City, are curiously absent.

Some of these cities are looking into new methods of land use, creating high-density shopping and residential areas that are bike- and pedestrian-friendly. Ideally, these communities will encourage people to travel without getting behind the wheel. Unfortunately, this isn't likely to help alleviate problems in the short-term. It will take vigilance and a willingness to make adjustments for these communities to have a real impact on traffic congestion in the future.

Reducing traffic congestion requires tough and sometimes unpopular decisions from the government level all the way down to the individual driver. As the problem increases, you'll likely see government officials look more carefully at their choices. As bad as traffic is in the United States, it's much worse elsewhere in the world. There's little doubt that American policymakers will watch what happens in other cities to see what might work in the United States.


Lots More Information

Related HowStuffWorks Articles
More Great Links

  • "A Toolbox for Alleviating Traffic Congestion." The Institute of Transportation Engineers. 1989.
  • "Introduction of traffic congestion pricing in Seoul, Korea." Asia-Pacific Environmental Innovation Strategies Research. April 29, 2003.
  • "Road Traffic and Air Pollution." It's Your Health. Health Canada. May, 2004.
  • "State of the Air: 2007." American Lung Association.
  • "Traffic pollution - measuring the real damage." September 21, 2005.
  • "Traffic pollution 'kills thousands every year.'" September 1, 2000.
  • Albert, Tanya. "Widening Roads Worsens Traffic Congestion." The Cincinnati Enquirer. January 13, 2000.
  • Arnott, Richard. "Alleviating Traffic Congestion: Alternatives to Road Pricing." Taxation, Resources and Economic Development (TRED) Conference. September, 1994.
  • Beaty, William. "Traffic Waves."
  • Bowlden, Terry, et al. "Building Better Communities: A Toolkit for Quality Growth." The Quality Growth Coalition. 2000.
  • City of Los Angeles Demographics - 1990 & 2000 Census
  • Malone, Robert. "Worst Cities for Traffic."
  • Morla, Ricardo. "Vision of Congestion-Free Road Traffic and Cooperating Objects. November, 2005.
  • Poole, Fiona. "Traffic Congestion." House of Commons Library. January 28, 1998.
  • Schrank, David and Lomax, Tim. "The 2007 Urban Mobility Report." Texas Transportation Institute. September, 2007.
  • Staley, Samuel R. "Do Highways Cause Traffic Congestion?" Reason Foundation. June 29, 2006.
  • Taylor, Brian D. "Rethinking Traffic Congestion." Access. October 1, 2002.