As said above, you are still flying under IFR during a visual approach.
So the controler must seperate you from all other traffics, which means keep you far enough acoording to his rules which are rather strict.
The pilot remains always responsible for collision avoidance, even if separated from other aircrafts.
Separate aircrafts means give them instructions so that the separation minimums will be achieved.It's the responsability of the controller, as you don't have the means to do that by yourself.
Collision avoidance means using all available ressources to avoid collision, that's always the crew's responsability
If you say "cancel IFR", then the controler still have to separate you from IFR traffic but not from VFR traffic in class C airspace.
That explains why from time to time a visual approach will be refused by the controller, because he thinks it will be difficult for him to maintain the separation minimums between you and other VFR traffic. In that case say "cancel IFR" and you will approach visually, and don't forget to log the last minutes of the flight under VFR.
Last edited by 172510; 21st Sep 2014 at 10:50.