This is parking lot of a shopping plaza at the junction of US highways 12/18 and 151, and that structure is a highway overpass approach. Highway 12/18 was routed to a bypass, called the Beltline, and punched through the southern extent of the city in the 1950's. It cut a residential street in half at about this location. (The obscured building behind it is located on that street.) In the 2010's the state Department of Transportation grade-separated the junction, hence the sloped approach.

The contemporary ads from the '50s tried to make this place look modern and glamorous, and maybe it was. We used to love coming here as kids, because it hosted a Toys R Us store. Now, the big store is a Home Depot, and the place looks even more sad and run down.

Through the magic of induced demand, the highway expansion predictably led to worse traffic congestion. There's no room to expand it more, so WisDOT plans to destroy another strip mall across 151 (out of frame to the right, no great loss), and also half a dozen more small businesses, maybe 20-30 houses, and a city park to build a free-flowing interchange. What's left of the neighborhood will be next to a freeway sound barrier, instead of a quiet, tree-lined street. (A bit to the west, some friends of mine used to have a view of the sunset from their back yard; now they have a concrete wall.) The Wisconsin legislature outlawed using eminent domain to acquire land for bicycle and pedestrian facilities, but by god, we're still happy to throw people out of their homes for cars. Freedom!

In many respects, too, we're lucky. The planners had the foresight to route the highways around the edge of the city, instead of punching right through its heart, as they did in so many other cities. (They did seize and bulldoze the Italian/Black neighborhood by other means, so don't fret. Racism finds a way.) On the back side of this shopping plaza was an Illinois Central rail line that went right downtown. There was a proposal to replace it with another highway, to provide a high-speed connection into the heart of the city for drivers. It's now a fairly nice bike path, actually.

Further, the city does have some genuinely nice spots, mostly in the old, pre-auto core, around the university campus. Which is weird, because we have examples of what's beautiful (for the wealthy people who can afford to live there), and then choose to build this kind of dreck just about everywhere else.

Not OP but that brownish structure looks like a highway barrier/ramp. I can hear this picture, that's how common this type of parking lot is in US