The Federal Whistle Blower Protection rules in the OSHA regulations protect drivers.  Here is another example of the rules in action:

OSHA has ordered New York-based Brindi Trailer Sales and Services to pay a fired employee more than $45,000 in lost wages, damages and fees after finding he was illegally terminated for notifying regulators of a workplace safety violation, Business Insurance reports. According to the report, the driver notified the company of defective equipment on his truck and requested the conditions be repaired. But the company refused, according to the report. The driver took the truck to be inspected and found it contained 16 violations. After repairs were made, the driver notified his employer and was discharged, according to the report.

Remember these rules apply to Federal Safety Rules and not necessarily to state rules like height, weight, length, and width.  Also keep in mind that actions by an employer need to be documented in order to be successful in court.