I think for your own sites, Google analytic, can give you traffic details. If it's someone else web page you can just see the SERPs and 'guesstimate' the traffic. I know that Alexa doesn't show individual page / sub domain traffic comparison.
I use a site called http://compete.com/ you type in your site vs your competitor site and they give you stats like unique visitors per month, all the keywords they use to drive traffic to there site, and so on... It's a nice free tool, but it is very, very costly to do an in depth search. The free version will give you the top 10 keywords they use. I would not recommend paying unless your are making a nice return. If you are it is a nice investment. This page will show you the cost which can exceed $500.00 a month. https://my.compete.com/plans/?_s_icmp=Z02i3Wb4