When it comes to highway accidents, NTSB doesn't have the staff to investigate anything more than the most high profile accidents
Actually, the NTSB investigates many highway, rail, and marine accidents that don't involve either fatalities or major property damage. The findings in many of these accidents are presented in a 'brief' instead of a full report. However, investigation of most accidents is discretionary. With lesser accidents, the NTSB tends to investigate only those where it is suspected the probable cause may be related to an item on their
Watchlist Most Wanted List (WatchList is Transportation Safety Board of Canada(TSB)'s critical items, not NTSB). The exception is aircraft accidents. As part of the legislation that created the NTSB, there is a requirement that the NTSB investigate every accident or incident involving a civilian or commercial aircraft, no matter how minor (yes, even the Cessna that lands at a small airport and veers off the runway onto the grass with no injuries automatically warrants an NTSB investigation). This mandate is one of the reasons why the NTSB staff is stretched so thin.