Why should my city not widen a highway?

Most likely, it won't work, and will come at a heavy cost, both direct and indirect.

In transportation-planning speak, the reason road widenings rarely alleviate traffic congestion comes down to two words: induced demand. Think of traffic less like a constant flow of vehicles, and more like a gas which expands to fill its container. If you create more road space, it "induces demand" for more driving. Some people will drive who otherwise would have skipped the trip or taken a different route. Some will drive at rush hour who otherwise would have chosen a less-busy time of day. Over time, more homes and businesses will locate farther apart from each other, to take advantage of the increased road capacity. Congestion will soon return to previous levels.

Want evidence to further understand this phenomenon or to help you convince others?

Large infrastructure projects like highway widenings are often accompanied by studies that purport to show great economic benefit. These studies are in fact misleading and pseudoscientific, and show vastly inflated, illusory benefits. These links explain why.

The downsides of widening a highway are many: a wider road means increased pollution, a more formidable barrier between neighborhoods, and of course the direct expense of maintaining and eventually replacing the roadway.

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