The most effective change would be to have a 3-5 foot (1-1.5m) passing rule. If it's not safe to go into the next lane to pass a cyclist, it's not safe to pass the cyclist at all.
(edit: and, if we're going to do the work to make people realize that they could be safer, make sure you're all the way past the cyclist before you pull in or turn right (left in the UK). Many motorists get their head past the cyclist and neglect the back half of their car, or the trailer they're towing)