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.
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.