To study the dynamic interaction between flexible pavement layers a special equipment was designed and built and a testing methodology was also developed. Theoretical analysis and initial testing have demonstrated qualitative aspects of the phenomenon. On such basis it was suggested to represent the horizontal interaction between flexible pavement layers as a structural restraint following Kelvin's model. The representing parameter has been defined by the authors as: Interlayer Reaction Complex Modulus Kl*. The statistical characteristics of the test developed were evaluated, together with the values of the norm (absolute value) and of the phase angle (between load and deformation) of the vector KI* at two different temperatures. Statistical analysis has shown that test reliability is adequate for research purposes but still low for normalization. The test and the parameter KI* appear to be suitable for the representation and evaluation of the interaction between asphalt concrete layers for flexible pavements.