From the faq.

The signage is good, and the vast majority of truck drivers notice the problem and avoid the bridge. Large signs alert driver to the low clearance several blocks before the bridge. Half a block before the trestle, a sensor detects overheight vehicles and triggers an LED blackout warning sign that was installed in May 2016. That same sensor also triggers a red-light phase at the traffic light directly in front of the trestle (installed in March 2016), so the driver has 50 seconds to read the warning sign next to the red traffic light and consider their next move.

For them to even be in this position, they have to ignore several signs, a flashing sign, and run a red light. Some people simply can't be helped.