Silly question...

You are a thoughful man for not wanting to have your family out there for hours waiting for you. 

I don't know if your race has live tracking but if so, that will give them the best indication to when you will cross the finish line. 

If they don't have that, you could put a cell phone in your T2 or SN bag and tell them to expect you in xx hours (your predicted marathon time). 

I think though if you do an honest assessment of your training times for long s/b/r you can predict your best and worst finish times and you'll probably fall w/i those times.

~Mike

IM FL did have some computer set up in a  tent where families can track athletes from IM Villa.

The problem with online tracking though is sometimes it's delayed a bit.  So there's a chance it may not be accurate.  I know when I did the race it showed me being in T1 forever- my parents were getting worried that something happened.  But it was just a delay with the timing system.

The best thing you can probably do is let them know your goal times (give them a time frame) that you'll finish each segment so they can be on the lookout for you.

Which race are you doing?

Silverman...

Yeah, the same old method should work, but it seems the window is bigger.  I could probably narrow a sprint down to 5 min.  An Oly to 10-15, an HIM to about 30.  An IM to about 60-90 minutes.  Not the end of the world I guess....

carry your cell phone on the run call them

or if you have someone youknow that is cheering someone else on have them call when you are an hour out from the finish
or could you have one person just be out on the run course to call ahead to the others.

It may be posted in the rules that one can not have an electronic communication device with them during the race.