Measurements of energetic proton (Ep ≥520 keV) intensities by The Johns Hopkins University Applied Physics Laboratory instruments on board the Voyagers 1 and 2 deep space probes, during solar flare-induced shock waves, indicate that the formation of solar energetic storm particle (ESP) events depends critically on the heliolongitudes of different locations on the large-scale shock front with respect to the source flare site. It is shown that large ESP ion intensity enhancements are observed for solar flare sites to the east of the spacecraft meridian, whereas only weak ESP events are associated with shock crossings where the flare sites are west of the spacecraft meridian. The results are explained in terms of the average ''interplanetary magnetic field-shock front'' configurations encountered by the Voyagers at various heliolongitudes of the solar flare-generated shock wave, i.e., quasi-perpendicular (quasi-parallel) shocks for eastern (western) solar flare sites. These configurations suggest that efficient energetic proton acceleration processes operate at quasi-perpendicular shock fronts consistent with the ''grad-B'' shock drift acceleration mechanism.