When compared to gasoline powered vehicles, LPG (light vehicles) emit about the same level of carbon monoxide. Results are better for larger heavier vehicles such as buses.
The primary safety concern is that LPG is heavier than air and tends to "pool" (in the event of a leak or accidental release). This makes it more susceptible to fires or explosions compared to other gaseous fuels such as natural gas (which is lighter than air and dissipates when released).
Propane (LPG) works best for heavier fleet vehicles relative to light vehicles primarily due to emissions. A primary focus of the DOE project above was to reduce the emissions of passenger vehicles - not larger heavier vehicles.