I do community theater, and punctuality is often an issue. These are the things I do to address it:
1) On the first night I ask everybody to put my number into their phones and to call me if they're going to be late. No matter what. Even if it's only a couple of minutes late. I explain that if we know you're late, it helps us decide how to handle the rehearsal. I then say, "and by the way, if you're late every day because of traffic, you need to leave for rehearsal earlier, because clearly the traffic is always going to be there." This usually gets a laugh.
2) At call time I call everybody who isn't there. Even if it's only one minute after the call time. I use the language that ChaCha suggested, to not be confrontative... "Hey, just checking to see how close you are?" or "Just calling to make sure you knew your rehearsal call was 7pm today..." Usually it only takes one of these calls for the person to start calling me when they're running late. (And if my director doesn't want me to do that, I won't. But I also won't apologize for rehearsals starting late for the duration of the show.)
3) Start on time, even if they aren't there. This is of course up to the staff to some extent, but you can usually do something that makes good use of the people who were on time.
4) I keep a sign-in sheet, and when somebody is late and calls, I note that. If they are late and they don't call, I note that too. If somebody is chronically late, I pull out my sheet and highlight their late days and show it to them, and say "hey, you seem to have an issue getting here on time. Is it traffic or what time you get off work, or what?" Usually I already know that it's just their pattern, but a personal talk and visual proof of the problem helps the conversation be taken seriously.
5) I also note and announce local events that are likely to cause traffic problems. If there is a football or baseball game, or a bridge closure, or anything that is likely to make traffic or parking horrific, I give them warning and tell them to plan extra time to get to rehearsal