IntroductionI started this post with the intention of finding some facts on whether or not a streetcar would be a waste of money compared to investments in highways. Even though I believe streetcars are a good investment for more than one reason, I wanted to address the congestion question directly, since it is seemingly the easiest question to get data on. (We shall see that I was in part wrong about this.)
About adding lanesScientific studies have shown that adding new lanes to highways does not necessarily reduce congestion; instead, adding lanes often makes congestion worse. This is often referred to as "Induced Travel".
The best and most neutral source I've found on this topic is at the Federal Highway Administration's website.
One way to think about this is to think to think of highways lanes as something similar to what economist call a "free good". What this means is that people will use up as much of the resource as there is. If more of the resource is added, people will just use that up as well. One way to change model is congestion pricing which makes the resource no longer "free".
Here is the big study OKI did on congestion in our region: http://www.oki.org/transportation/congestionmgmtanalysi.html (direct link to PDF)
If you turn to page 30 (page 36 of the PDF), you'll see OKI's evaluation of many strategies for reducing congestion including congestion pricing, rail transit, and additional lanes. If you look at table 10-1 (page 39 of the PDF) you'll see an analysis of which strategies could be used at which locations across the county. You'll see that on I-75, both rail-transit and additional lanes are give black squares which indicates high-cost/high-impact. (I don't see any way of knowing what ranges of costs are represented by the shapes. Perhaps a $100 million streetcar and a $800 million highway project are both squares.) There are also a number of X's which I'm not exactly sure how to interpret. I'm guessing either the analysis wasn't done there, or that strategy is infeasible for that location. If you turn a few more pages, you'll see that I-75 widening to four lanes in both directions is listed as an underway project.
More about the transportation projects in this particular corridor can be found at http://www.i75millcreekexpressway.com/ and http://www.thruthevalley.com/.
About streecarsI tried browsing through here for more info on what OKI has published relating to the streetcar, but couldn't find anything showing it's estimated effect on highway congestion (or any road congestion at all). Mostly I'm finding cost esitmates and ridership estimates.
There are some up-to-date presentations on the city's website here but there isn't a whole lot on the congestion question there either. Again these focus more on cost and ridership. One study showed impacts on local traffic, that is to say traffic on the sames streets as the streetcar runs; but they didn't mention impacts on regional traffic.
ConclusionI'm guessing that the reason that the question I originally set out to answer hasn't been answered is that the effect of a streetcar on highway congestion is probably pretty small. The number of people that forego a car trip on the highway to ride a streetcar instead are probably small to non-existent. Travel between areas served by a streetcar is probably already accomplished without using highways.
Thus, I've come to the conclusion that at least in terms of their effects on congestion comparing highway investments with streetcar investments is probably like comparing apples to oranges.