Portland believes, and they are probably correct, that light rail would reduce the number of cars crossing the bridge. Who would drive in order to pay lots of money for parking if there was a reasonable alternative?
Most places in Portland have free parking. Most of the people crossing the bridge aren't going Downtown.
Wrong and wrong.
Portland's public garages range from $12/15 at the city-owned garages to $40+ for some garages in downtown. The only free garages shown on
Parkopedia and
ParkingKitty are those at MAX stations.
While I can't find exact numbers, we can make some assumptions based on published data from Metro:
50% of Clark County's workers (about 87,500 commuters) leave the county for work, and the bulk head to Multnomah County. Seeing as
3/4 of all jobs in the Portland metro area (p. 25) are in Downtown Portland or the 3-mile radius around it (all within the light rail and streetcar catchment area), one can assume that most of those Multnomah-bound Clark commuters are indeed heading to Downtown Portland.
It's not all of the bridge traffic, but makes up a relatively large percentage of drivers during peak periods.
A bonus fun fact:
10 to 20 percent of pre-pandemic traffic over both Columbia River bridges were attributed to people living in Washington and shopping in Oregon to avoid sales tax. Simply tolling the crossings would cut this traffic significantly and even encourage retailers to open on the other side of the river, thus providing greater economic opportunity.