Worst-case scenario, maybe the boss is "letting it go" but one day, without warning, when "enough is enough", the company will let you go†, for a number of minor things including "all those times you were late"...
†Before anyone argues that "they can't do that" (ie., fire without prior warning), real-life proves otherwise. There are ways around every employment-related law or policy, and I've seen it happen. For example: paying out severance in lieu of notice/warnings. Besides, you don't want to me that person that's still employed only because "they couldn't legally fire you".
My suggestion is... Stop Being Late!!
The company has already made a special arrangement for you with your shortened days, and you're pushing the boundaries of the agreement. The boss might feel awkward confronting you about this, or maybe feels sorry for you, but either way, there is a limit, that you get closer to each time you're late. If it's happened several times, then, believe me, someone's noticed.
Your correct course of action really depends on how often you're late, and of those, what portion is because you slept in, as opposed to something that can be verified, like unusually high traffic.
If the traffic is indeed the problem most times, and it's more than once a month, then you need to make alternate arrangements with your employer to allow even more flexibility than you've already been given, and ideally you will be able to make up the missed time at the end of your shift or on an extra day. "Hopefully" not meaning "so you don't lose pay" but rather, so you can produce the amount of work that was agreed upon when you were hired.
Another thing to consider is how unfair this is to your coworkers, who all have their own life responsibilities and deal with the same traffic (or worse, by the sounds of your schedule) but they manage to get to work on time every day. You're already arriving after them, leaving before them, and on top of that, your actions are disrespectful to them. Are you caught-up on every single project, or are there, by chance, others waiting on your work? It's very impolite and inconsiderate, and it wouldn't be surprising if they are already discussing this when you're not there, and bringing their concerns to the boss... even if the boss doesn't take action based on that, you're still better off being on your co-worker's good side.
Don't think for a second that if you're late because of traffic "it's not your fault". It is up to you to be at work on time, including managing your time in such a way to allow for normal things that can happen in any given day, like unpredictable traffic. At least come up with a better excuse (unlike these).
If the traffic is the issue, leave home earlier, every single day. Be "that worker who's always early" instead of "that worker who's always late". (Which one has a nicer ring to it? ...and which one is more likely to lead to job-related benefits like higher pay or a good reference letter?)
If you often have trouble waking up, get a second alarm clock, preferably one that requires more than a button-smack to shut it off. There are "smart wake up apps" that requires you to do tasks, like solve a puzzle, or "take a picture of something blue" before the alarm will be disabled, or better yet, there are clocks that "run away from you" when you try to shut them off...