It can actually be worse. This is called Braess's paradox [1]. The relevant part is: "an extension of the road network may cause a redistribution of the traffic

It can actually be worse. This is called Braess's paradox [1]. The relevant part is: "an extension of the road network may cause a redistribution of the traffic that results in longer individual running times." Further down in the Wikipedia article is an actual example. [1] https://en.wikipedia.org/wiki/Braess%27s_paradox https://en.wikipedia.org/wiki/Braess%27s_paradox