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Researchers in Denmark found that municipalities underestimate the economic and environmental impact of new roads.

Here's an all-too-common phenomenon - a city decides to build or expand a road to decrease traffic. The road is built. Traffic increases instead. This is called induced demand, whereby drivers avoiding the road because of congestion jump back onto it once the project is finished.

But municipalities often fail to properly account for this phenomenon, according to a new study of out Denmark.

The researchers found that "the traffic-generating effects of road capacity expansion are still often neglected in transport modelling." This can lead cities to make mistakes when assessing the environmental impact and economic vitality of proposed road projects.

As Streetsblog writes:

According to the study, completed by researchers at the Institute of Transport Economics and a Danish university, this leads to skewed cost-benefit analyses that call for new highways and road widenings of dubious benefit to the public.

Researchers reported that perceived time savings make up the largest portion — sometimes 85 percent — of the economic benefits assigned to prospective highway projects. But an unanticipated boost in traffic volume can turn many projects that would theoretically pass analytical muster into economic losers. Unless transportation agencies are carefully accounting for these effects, however, many of these projects get built anyway.

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