The biggest problem is that controlled airspace DOES NOT stop collisions, as far as traffic lights don't stop them. You still have to have the human element make a decision as to compliance with instructions. Planes and cars will rarely fly into each other on purpose, they collide when they think it's clear around them, have a little box squawking at you when things get close and you take notice. ATC gives you a false impression of safety because everyone should be doing as told, which means you are perfectly set up to have problems when they don't, ie reduced lookout, because I assume someone else has made the path clear. As I said earlier a PA-28 almost collided with an ATR in Albury airspace in plain view of the tower, but only TCAS alerted the ATR crew that there was an aircraft in proximity and prompted a go-round. Also the Metro vs the Cirrus in the US recently, at a controlled airport. Mandate carriage and use of transponders to all mandatory radio areas, TCAS/ACAS is the only thing that will make significant difference without severely limiting airspace.