This is 99% Invisible. I'm Roman Mars. Detroit, Michigan is the home to the Ambassador Bridge,

which links Detroit to the Canadian border city of Windsor. The ambassador is essential. CNN once

called it the most economically important one and a half miles of roadway in the Western Hemisphere.

Every day, endless lines of trucks wait for hours on a concrete plaza flanked by rows of traffic cones

so they can be processed by a border agent.

Best producer, Andrew Laypen.

The ambassador is the linchpin of the North American auto industry.

Cars are constantly being driven back and forth to different factories.

Last year, more than 2 million trucks drove across the ambassador.

In fact, 30% of all trade between the U.S. and Canada rolls over this bridge.

Everybody has to use the bridge because they basically don't have any

other choice. There's a tunnel nearby that's too small for most trucks to drive through, and the

next closest international bridge is some 65 miles away. To get to the bridge, all those freight trucks

first have to drive by a residential neighborhood in southwest Detroit, called Mexican Town.

And for many years, they had to drive right through it.

Mexican Town is right at the bridge's entry point, and residents have suffered through loud honking and negative health effects for decades.

Here's how Sam Butler, president of a local neighborhood group, described it to me.

There is this black soot on windows or on the outside of people's homes that face the bridge,

that you can literally take your finger, wipe off a piece of this black soot, living so close

to a international crossing that gets

10,000 trucks per day.

Folks in Windsor don't like the bridge either.

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