Beyond the insurance they're required to carry under federal law (typically $750,000 or $1,000,000) it is not uncommon for small trucking operators to be essentially judgment-proof when sued after an accident. So it's on with the search for deep pockets:

For example, there have been an increasing number of claims against freight brokers for injuries sustained in an accident with the motor carrier to whom the broker tendered freight. More recently, at least one published opinion has recognized the possibility of such a claim against a shipper who directly obtains a motor carrier.

The claims have required stretching and reworking common law rules that ordinarily would bar such claims, including the rule "that one cannot be held vicariously liable for the acts of his or her independent contractors". The article is Robert T. Franklin, "But I Didn't Do It!" Expanding Theories of Vicarious Liability, 58 Fed'n Def. & Corp. Couns. Q.347 (2008)(PDF) (via Day on Torts)