We use Galaxy Zoo 2 visual classifications to study the morphological signatures of interaction between similar-mass galaxy pairs in the Sloan Digital Sky Survey . We find that many observable features correlate with projected pair separation ; not only obvious indicators of merging , disturbance and tidal tails , but also more regular features , such as spiral arms and bars . These trends are robustly quantified , using a control sample to account for observational biases , producing measurements of the strength and separation scale of various morphological responses to pair interaction . For example , we find that the presence of spiral features is enhanced at scales \la 70 h ^ { -1 } _ { 70 } kpc , probably due to both increased star formation and the formation of tidal tails . On the other hand , the likelihood of identifying a bar decreases significantly in pairs with separations \la 30 h ^ { -1 } _ { 70 } kpc , suggesting that bars are suppressed by close interactions between galaxies of similar mass . We go on to show how morphological indicators of physical interactions provide a way of significantly refining standard estimates for the frequency of close pair interactions , based on velocity offset and projected separation . The presence of loosely wound spiral arms is found to be a particularly reliable signal of an interaction , for projected pair separations up to \sim 100 h ^ { -1 } _ { 70 } kpc . We use this indicator to demonstrate our method , constraining the fraction of low-redshift galaxies in truly interacting pairs , with M _ { * } > 10 ^ { 9.5 } M _ { \sun } and mass ratio < 4 , to be between 0.4 – 2.7 per cent .