Natural collision coordinates and a zeroth-order vibrational-adiabatic approximation are used to treat linear reactive collisions. Nonadiabatic effects on barrier transmission and on vibrational state of products are calculated. The present results are classical and are compared with exact classical numerical results for the H+H2 reaction in the range 7-20 kcal/mol of initial relative translational energy. The agreement is encouraging and the results support the concepts introduced earlier of statistical adiabaticity and of nonadiabatic leak. At low energies the reaction is adiabatic on the average (initial vibrational phase average), thus justifying activated complex theory for this system. The relative importance of reaction path curvature and of vibrational frequency variation along the reaction path in inducing nonadiabatic effects is described. Implications for a quantum treatment, activated complex theory, and highly nonadiabatic systems are noted.