Hazmatspotting is one of my hobbies: when I see a truck with a hazardous-materials placard, I check the number against the Emergency Response Guidebook (I keep a list on my PDA, adding details about when and where I first spotted it).  This truck is carrying diesel fuel, although #1993 is a bit of a catchall -- it can include "liquid disinfectant, n.o.s. (not otherwise specified)", "cosmetics, n.o.s.", "drugs, n.o.s.", etc.  This is okay because you'd respond to a spill of any of these things in roughly the same way.
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/44124401501@N01">Ben Ostrowsky</a> from Atlanta, United States
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/sylvar/447515/">Diesel truck</a>