If I understand what you’re asking for, assuming that you also have SmartThings (this forum is for people who are using the Samsung SmartThings ™ home automation platform), this is easy. 
- Connect the driveway sensor to any open/close sensor that works with SmartThings.
See the following FAQ for some typical devices of this type. (The topic title is a clickable link.)
These devices will then show as “open “ to SmartThings when the relay is activated. And smartthings will pass that information along to Alexa.
- when that sensor shows as open, use that as the
“When this happens” in an Alexa routine (not a SmartThings routine)
You can have the action in that routine be having any or all of your Alexa devices make a specific announcement. Or you can have them play from a set of available sounds, including several different chimes. You create this routine in the Alexa app.
So it should be very straightforward.
https://www.amazon.com/gp/help/customer/display.html?nodeId=202200080
If by chance you aren’t using the Samsung SmartThings system, you would need to find any standalone Wi-Fi open/close sensor that works with Alexa that can act as a dry contact and you should get the same result. I just don’t happen to know specifications for those since, again, the assumption is that all of the questions and answers in this particular forum are for people using Samsung SmartThings.
Alexa routines can be triggered at the present time from an open/close sensor, a motion sensor, or a lock. So that’s what you need as the trigger.