Traffic usually doesn’t decline with transit. The people who are making car trips today are driving because that makes sense for their trip origins/destinations.
I wrote something about this before here:
What changes with TOD is that future growth in traffic slows (and eventually reverses). New people making new trips find origins/destinations (houses, offices, schools, shops) for which it’s more convenient to take transit / walk / bike than to drive. It’s not changing the lives of people today (hence they shouldn’t fear it!), it’s changing the lives of people tomorrow.
Giving those people tomorrow more choices means that they won’t be driving on the same roads as you. And you’ll have more travel choices, too.
Keep in mind that a huge proportion of the driving reduction that does take place (and not decline per se as much as avoided increase) isn’t substituted 1:1 by transit trips. People in transit-rich places take transit more, but they also walk and bike more, and when they do drive it’s for much shorter distances.