He was a Field Artillery Officer and was "officially" on his way back east after finishing post-graduate work in Monterey, Ca where he was studying business and public policy.
He was newly assigned to the U.S. Army Testing and Evaluating Command Center.
I'd venture a guess that he was either working on something shady or was transporting information that the Pentagon deemed sensitive or secret.
I'd also venture a guess that this Captain Clark was either an Army Officer....but I doubt that was his real name....or maybe even a spook for the CIA, NSA or DIA.
He was being assigned to the Army Evaluation Center Fire Support Evaluation Directorate.
He was an arty officer so this makes sense.
His studies in Monterey at the Naval Post Graduate School were done in order to secure his Master's of Science degree, with distinction, in Systems Acquisition Management.
He must've had some information with him that the Army didnt want falling into the wrong hands...probably nothing more.
If he were a high mucky muck spook then I doubt he would've been "driving across the country" with sensitive documents...they would've flown him on a "Tower Air" jet or some other MAC flight.
He probably just had some fire support documentation that was a little too high speed to allow it to remain out in the open.