Traffic congestion reconsidered

Hence, building new roads or adding more lanes

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to existing ones might seem as obvious solutions. It surely sounds logical enough: as cities grow bigger, roads serving them should follow the same trend. Drivers should have more space to move if they have wider or new roads, which relieves congestion and makes cars go faster. This argument is frequently used by government officials to elaborate on the importance of new and frequently costly infrastructure projects.

However, it hasn't always turned out this way in practice, and the reasons behind could be found in long-term effects of the induced demand. Induced demand refers to the situation where as supply of a good increases, more of a good is consumed. This implies that new roads essentially create additional traffic which in turn causes them to become congested all over again.

Why does this happen? After a highway is widened, initially there are fewer traffic jams and trips become quicker. However, these improvements change people's behaviour. Drivers who had previously been avoiding that route because of its congestion now consider it as an attractive choice. Others who previously used public transportation, bicycles or other modes of transportation may shift to using their cars. Some people might change their time of travel—instead of travelling in off-peak times to avoid congestion, they start traveling in peak-times, increasing congestion. Hence, as more people start using the widened highway, initial time-saving effects are reduced and eventually disappear.

https://phys.org/news/2019-06-traffic-congestion-reconsidered.html