Another idea is a change to the height detector warning.
Instead of blinking lights, something like a railroad crossing barrier, but one that will fire very quickly and also not cause damage to a car that does not stop (in case of erroneous firing)?
Maybe even better to make it so high that normal consumer cars will just go under, anyway. And only trucks will get the message right quick when it drops 3 feet down from the barrier all glowing with "STOP! Do not enter."
It seems the blinking red lights don't work, because to everyone who drives it means stop and look for traffic... then go. Not stop and stay stopped and then turn around.
To say there's no way to get the attention of a driver that isn't paying attention is not thinking it through. If he is not paying attention to visual cues, he would have ran off the road and crashed a long time ago. He can't get here if he is not looking at something... something more important to him than reading a sign. If he see's a truck coming at him, he will react. He is looking and processing visual information. Maybe there's a way to reach this driver, visually, then.
I have a personal anecdote from an 8 hour drive. There was some road work and lots of traffic, going on miles and miles and miles. To me? W/e. I was 4 hours of driving at this point, and I'm just following the car ahead of me. This traffic and signs went on for like an hour, and I read... none of them. Well, I'm in the left lane. Finally, the car ahead of me merges right, and I see.... road block coming up FAST. I have plenty of time to stop, still. But signs, lights, warnings, nothing do I care.. esp if you have warning that you must merge an hour before you get there. This is crying wolf. you get desensitized. Traffic going this way, I'm going this way. Ok, done thinking. Actual physical road block?.. ok, I see this and I know it is important. No more warnings. This is what the last hour of signs was all about, and it is obvious. It's time now to give a shit. But if I stop, I will be stuck for hours until traffic dies off. So I look, I have a spot, I merge...in front of a unmarked off duty cop, who apparently had a bad enough day to take his personal time to play traffic cop to me. Ticket for passing in a no pass zone. (And he didn't believe my honest explanation; to him, I was weaving in and out of traffic and cutting people off to "beat" others when I was actually following same car, same lane for an hour.)