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We use a sample of 19 Gamma Ray Bursts (GRBs) that exhibit single-peakedoptical light curves to test the standard fireball model by investigating therelationship between the time of the onset of the afterglow and the temporalrising index. Our sample includes GRBs and X-ray flashes for which we derive awide range of initial Lorentz factors ($40 < \Gamma < 450$). Using plausiblemodel parameters the typical frequency of the forward shock is expected to lieclose to the optical band; within this low typical frequency framework, we usethe optical data to constrain $\epsilon_e$ and show that values derived fromthe early time light curve properties are consistent with published typicalvalues derived from other afterglow studies. We produce expected radio lightcurves by predicting the temporal evolution of the expected radio emission fromforward and reverse shock components, including synchrotron self-absorptioneffects at early time. Although a number of the GRBs in this sample do not havepublished radio measurements, we demonstrate the effectiveness of this methodin the case of {\it Swift} GRB 090313, for which millimetric and centrimetricobservations were available, and conclude that future detections ofreverse-shock radio flares with new radio facilities such as the EVLA and ALMAwill test the low frequency model and provide constraints on magnetic models.

GRB 090313 and the Origin of Optical Peaks in GRB Light Curves: Implications for Lorentz Factors and Radio Flares