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What is Road Capacity?

Mary McMahon
Mary McMahon
Mary McMahon
Mary McMahon

Road capacity is the maximum potential capacity of a given roadway. It can be expressed in terms of vehicles per hour or per day. There are numerous limitations on road capacity which make it highly unusual for roadways to actually attain their stated capacities, and in some cases there have been efforts to constrain road capacity for the purpose of limiting traffic to reduce congestion or environmental problems.

When road capacity is calculated, traffic engineers assume that there are no limits on how the road can be utilized. All of the lanes on the road are open, for example, and limitations such as weather which could hamper driving conditions are not factored in. Given the speed a road is rated for and including factors such as stops for toll collections, engineers can determine how many cars the road should feasibly carry.

Concerns about the environment may lead to traffic restrictions in some regions.
Concerns about the environment may lead to traffic restrictions in some regions.

One of the biggest limitations on road capacity is drivers. Drivers are not automatons, and they do make mistakes. Inattentive driving, speeding, driving recklessly, and other activities on the part of users of the roadway can slow overall traffic. Likewise, failure to merge in an orderly fashion, frequent exits and entrances from a roadway, and other activities can cause traffic to slow. Most traffic engineers note, for example, that traffic around interchanges slows considerably solely because of drivers. Mixed vehicle use can also become a factor in road capacity.

Other factors can include environmental issues. Roads in rural areas may have slower traffic as a result of crossing animals, and in some regions concerns about the environment may also lead to traffic restrictions. In parks, for example, it is common to limit annual vehicle traffic so that it does not become too disruptive to animal life. Safety of children may also be a worry along some roadways, making it impossible to utilize a roadway to the fullest without endangering people.

New roads under development are also assessed in terms of potential capacity. Engineers want to confirm that a road will bear the estimated traffic it needs to carry with room for increasing traffic and expansion, since traffic burdens are on the rise in most regions of the world. Engineers may also consider issues such as deliberate throttling and limiting of traffic to control for congestion and other issues. For example, road capacity is meaningless if there is not equivalent parking capacity for all those cars to use, and so a city might encourage people to carpool and use public transit in order to reduce the demand for parking.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a WikiMotors researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Learn more...
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a WikiMotors researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Learn more...

Discussion Comments


@discographer: As a traffic engineer, I could say that increasing capacity in urban areas usually turns out to be not very effective in time. In my opinion, public transit and bicycles are the key solutions for today's problems. Make the car smaller and also increase their occupancy and you will also have a stable solution.


I never really thought about all the factors that road engineers have to take into consideration to determine what the road capacity should be for a particular road. Weather factors are an important one. Rain can really slow down traffic, especially in a rainy area. Even the sun at certain angles can interfere with visibility. I'm surprised that weather conditions aren't factored in.

It's seems like it is the actions of the drivers on the road that really affect road capacity. On some roads, different types of vehicles can cause a problem. For example, large trucks, especially up hill can slow traffic and on some roads, bikes can definitely slow down traffic as drivers slow down to try to anticipate what the bicyclist is going to do.


In the first place, most cities need to plan for higher road capacity because it's clear that most people are not going to give up their cars. Unless commuters have a really convenient public transportation route, like almost door to door,they're not likely to use public transportation regularly.

With the need for increased capacity, highway planners need to take into consideration that the roads that are running at high capacity become in need of repair quite often - potholes being one of the worst problems. I guess we need to build more highways.

Public transportation is still important as there are quite a number of people who can't or don't want to drive for a variety of reasons.


@ceilingcat-- Actually, I think both of those methods are used to measure road capacity. This is what I remember from my conversations with my brother-in-law who is a traffic engineer.

Traffic engineers can measure capacity by marking a point on the road and counting how many cars pass that point in a given period of time, for example how many cars per minute. They can also count the number of cars a certain length of road can hold, like a mile. If they use this method, I think they count cars per mile per lane and then multiply by the number of lanes in that road.

I personally don't believe that road capacity should be taken very seriously. Because like the article mentioned, it's not a definite measurement. We can test road capacity as much as we'd like but since we can't guess how cars are going to be behave, it's never going to be exactly right. It's just an estimate, it's something to work with and that's all.


@alisha-- Yea, road capacity is very important. If the capacity is not large enough for demand (the number of cars traveling), it results in congestion or dense traffic.

Driving in a low-capacity congested road doesn't just waste our time, but we also use more gas, create more pollution and have a higher chance of getting in a car accident. We could also consider the stress of driving in a congested road as a disadvantage.

I agree with you that roads should be tested for capacity and widened if that road experiences congestion recurrently. The examples you gave about rush-hour congestion and congestion on the weekends are recurrent congestion, so it needs to be resolved.

Sometimes there is an unexpected situation though, like a car accident or an unplanned road maintenance. We can't obviously increase the capacity of a road at the drop of a hat, it costs a considerable amount of money to widen and build more roads. So not much can be done for unexpected congestion.


I would think that drivers using the roads at capacity or close to capacity would result in a lot of damage to roadways. I know the roads near where I live aren't used to capacity by a long shot. However, they seem to always be in need of repair.

They always have pot holes or need repaving or repainting. I assume this is because of regular wear and tear and weather conditions. I can only imagine how bad it would be if there were a lot more cars on the road.


@ceilingcat - Interesting. I've actually read about efforts in some cities to constrain road capacity and encourage people to use public transit.

Cities do this by limiting the amount of parking spaces, for one thing. No one is going to drive somewhere if there isn't anywhere for them to park once they get there.

Also, I read that they can also constrain the road capacity by closing some lanes. Fewer lanes means fewer cars can use the road at the same time.


I think the whole idea of road capacity is kind of interesting. For instance, are the calculating road capacity at a point when traffic is flowing? Or just how many cars the road can hold?

I think it definitely makes a difference. If you can't really drive down the road at the speed limit, I think that I would argue that the road is over its capacity. Roads are intended to carry traffic, not become parking lots during rush hour.

I think this is a big problem in a lot of place near me. The public transportation around here is pretty awful, so people don't have any choice but to use the roads. I think if they improved the public transportation, the road usage would fall under capacity again.


I think the road capacity in all the major cities in the US need to be increased. The roads are not large enough to hold all the cars that are out and about during rush hours and holidays.

I live in the Washington DC area and have lived in Chicago and New York before as well. The highways going out of the city are always jam packed on the way home from work. It's even worse when it's a three day weekend or a holiday.

I once remember spending over an hour to get to where I was going that normally takes 10 minutes. I think road engineers need to develop roads based on the capacity that would be needed during heaviest traffic times. Otherwise driving around the city becomes a nightmare.

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    • Concerns about the environment may lead to traffic restrictions in some regions.
      By: Dave Allen
      Concerns about the environment may lead to traffic restrictions in some regions.