Of all the brokers I've ever used, IB seems to be the most transparent in how they route orders and how their smart-router works if you choose to use it. They have several sub-webpages dedicated to explaining how it works and what your choices are in routing orders and selecting different algo-types. Disclosure far far far above and beyond any other broker I'd ever used. (Of course, my sample size of brokers used doesn't include all brokers in existence, so there are probably better ones.... but all I'm saying is, as far as I'm concerned, IB is the best I've tried thus far in terms of execution and price improvement.)