A Corporate pilot reported taking evasive action on IAD Taxiway Alpha 4 after a Service Utility Van made a left turn onto the taxiway without notice.

Narrative

After landing on Runway 1C at IAD and clearing runway at YANKEE 6 Ground advised us to taxi via YANKEE to ALPHA and hold short of ALPHA 1 and contact Ramp Control. After contacting Ramp Control; I was cleared to taxi to Taxiway BRAVO via ALPHA and ALPHA 5. Approaching Taxiway ALPHA 4 I noticed a service utility van on my left side parallel to me on the main terminal ramp north of Taxiway ALPHA. The vehicle then made a right turn onto Taxiway ALPHA 4 without slowing down or showing any sign of stopping before entering Taxiway ALPHA 4 and did not stop prior to starting to cross Taxiway ALPHA. I was at a slow taxi and applied the brakes to a full stop to avoid the vehicle which appeared to be proceeding to cross Taxiway ALPHA just in front of me. I realize that airport operations are very intense and all parties must be under constant attention and aware of their surroundings. As an aircraft operator I have always maintained a sterile cockpit on ground and after takeoff through 10;000 FT. Possible airport vehicle operators should be advised of the same procedures when operating on ground and taxiway areas.