I am doing a regression analysis on the various factors which influence accident levels in my city. The 2 factors used in my regression model are covariates :

i) `urbanization` level of the city and,  

ii) the `percentage of persons buying cars` within the past year in the city.

From my regression analysis, both showed significant positive association with 
my outcome variable.

However, when I presented my regression model, one of my colleagues mentioned that my analysis, i.e., the impact of the 2 covariates on the dependent variable could be flawed and does not reflect reality since I have left out an important covariate such as the `number of alcohol outlets/pubs in the city`. What statistical reasoning can I rebuttal his argument with? I am not quite sure what to make of his argument and if it is even valid. Suggestions/discussion are welcome. Cheers.