Trucks carried 71.7% of freight, measured by value, in the US in 1997, according to the Commodity Flow Survey just released as part of the 1997 Census of Business.

The survey of freight originating at 100,000 businesses provides the most comprehensive database ever available on freight by mode of transportation, commodity, shipment weight, shipment distance and origin and destination. It excludes farming, government, services and retail.
Want to know the tonnage of bakery products shipped by for hire trucks in 1,000 to 10,000 lb shipments for a distance of 250-500 miles from Illinois to Missouri? You can find the answer in the database. Within a few months, 50 metro area databases will let you ask the question for Chicago to St. Louis shipments.
The national, state and (soon) metro databases are available from the Bureau of Transportation Statistics at 202/366-5685. Or you can download them from http://www.census.gov/econ/www/cfsmain2.html