The ornamental design for a trafficpostbase, as shown and described.
FIG. 1 is a top perspective view of the traffic post base showing my new design;
FIG. 2 is a front view thereof, the side and rear views being identical; and,
FIG. 3 is top view thereof.
Portions or features of the traffic post base not shown in the drawings form no part of the claimed design. The broken lines in the drawings form no part of the claimed design.