If the 2003 work-hour rule for truckers were detrimental to safety, the Federal Motor Carrier Safety Administration says it "would expect to see significant increases in the number of fatigue-related crashes as well as fatalities and injuries." Because it hasn't seen the increases, it assumes safety isn't being compromised.
Yes it is, and the safety consequences are apparent from the appropriate data. Deaths in most kinds of crashes have declined in recent years, and the appropriate question isn't whether they've increased in truck crashes but whether deaths in truck crashes have declined in line with deaths in other kinds of crashes. They haven't.
The passenger vehicle occupant death rate in crashes with large trucks decreased 8 percent during 2003-07, from 144 to 133 per 10 million registered passenger vehicles, while the rate in crashes with other passenger vehicles instead of trucks decreased 28 percent (from 456 per 10 million to 328). During 2003-06 truck occupant deaths increased from 32 to 35 per 10 billion vehicle miles. Thus, deaths in truck crashes haven't declined as much as would have been expected in the absence of changing the federal work-hour rule.