Of course, there will be some variance and every situation is different. I don't know about the route you're referencing, but one factor is the availability of alternative routes. Obviously, if you're talking about a place that only has one or two options and a fairly steady traffic flow, adding extra lanes may indeed reduce traffic. A place like a downtown city where there are hundreds of roads and numerous exits that lead to the same place, you're likely to see much less variance. If you're legitimately seeing more people entering downtown to a breakpoint [[where the infrastructure is legitimately not built to handle it), then adding infrastructure will help. However, as has been mentioned, miles driven in the area is generally down, and Detroit surely does not lack roads. When M-1 rolls around, we'll see even less auto traffic.
It's quite possible that this will be a net-positive for the public, but I think the chances skew toward neutral at best. The main benefactor would likely be Gilbert/Greektown. Now, since they're paying for it and that area likely will need to be rebuilt anyway, I don't have a huge beef with it, but I do think it seems a bit short-sighted. One thing I do worry about is that it gives Gilbert & co. a reason to oppose "boulevarding" 375, which I am in favor of.
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