An experimental investigation of the lid-driven cavity flow of a Boger fluid is completed in order to verify the scaling theory for the aspect ratio dependence of the critical conditions for elastic instability as proposed by Grillet, Yang, Khomami and Shaqfeh [A.M. Grillet, B. Yang, B. Khomami, E.S.G. Shaqfeh, Modeling of viscoelastic lid driven cavity flow using finite element simulations, J. Non-Newtonian fluid Mech. 88 (1999) 99-131]. Measurements of the critical conditions in shallow cavities show that the critical Weissenberg number and the wavelength of the instability are independent of the cavity aspect ratio, Lambda, for aspect ratios less than unity. Our experiments also demonstrate the importance of polymer concentration on the stability of cavity flows. The effect of concentration is included in new scaling arguments based on those developed for Taylor-Couette and Dean flow elastic instabilities; thus, we successfully describe experimental results at different polymer concentrations. Further experiments are described using flow-induced birefringence to measure the polymer stresses prior to the onset of instability in order to investigate the role of the downstream stress boundary layer in the stability of the flow. Measurements taken in three aspect ratios provide evidence that, for shallow cavities, the polymer stress emanating from the downstream corner can reach the region of curved streamlines at the bottom of the cavity, and possibly result in an elastic instability. This is consistent with the scaling theory proposed by Grillet et al. [A.M. Grillet, B. Yang, B. Khomami, E.S.G. Shaqfeh, Modeling of viscoelastic lid driven cavity flow using finite element simulations, J. Non-Newtoniam Fluid Mech. 88 (1999) 99-131].