From my knowledge, Arizona has had a bit of a problem with freeways in the past, particularly in the Phoenix metro area. With the city being so new, PHX was just beginning to build its freeways just as the government was slowing down on giving that sweet x10 matching for interstate highways, and that presented a problem for the new city. If you look on a map you'll notice that AZ has no auxiliary interstate highways due to its late joining to the US, and the need to complete I-10 and I-17 through the city. Phoenix, while not nearly as much now, still struggles slightly with this problem today. Their solution to this lack of funding from the government was to skip 3DI's all together and make all the ones they needed "Arizona State Highway Loops" to bypass federal approval and get the roads they needed, which came about as the Loops 101, 202, and 303 all incompletely circling the city. When I go to college, this and truck traffic are going to be the two main problems I want to help work on in addition to already existing congestion in other cities around the country.
Interstate Highways
I-10 - Papago Fwy
I-17 - Black Canyon Fwy
I-19
I-40
US Routes
US-60 - Superstition Fwy
US-180
State Routes
AZ-64
L-101
L-202 - Red Mountain Fwy
L-303