help with travel time shortcut

how do i make an automation that takes the travel time to go to an event on my calander and adds15 min(or whatever time i desire) and then give me a notification to leave when it’s that time.

so say if it takes 15 minutes to drive somewhere. it takes that time and addds 15 minutes and so 30 minutes prior to the event it tells me to leave. i want it to be dynamic so the travel time updates if there’s traffic or anything.