Stop what you're doing and call them right now - don't wait until you're closer or think you might make it. The reason this matters more than just letting them know is that restaurants actually start prepping for the next seating as soon as your reservation time passes, and once they've mentally moved on or started giving tasks to the kitchen staff, getting your attention back is harder than you'd think.
Most places will absolutely hold your table for 15 minutes if you've given them a heads up, even on Friday nights. The difference between a "hey, I'm running late but will be there at 7:15" call and just showing up late is pretty massive - one shows respect for their operation and the other looks like you forgot entirely. When you call, be direct about your ETA and apologize briefly, that's it.
Since it's a Friday and likely packed, they might try to seat your table to someone else if the timing drags on, so call sooner rather than later and give yourself a little buffer in your ETA estimate just to be safe. They're way more willing to work with you when you're communicating than when you're silent.