Following research by Dr Ian Walker of Bath University which concluded that drivers give yu less clearance if you wear a helmet

a recent experience of mine suggests a better alternative!

Myself, my wife and my son cycled to a pizza restaurant to celebrate his 10th birthday where we were give silver helium filled balloons to take home with us.
After the meal, we decided to tie these to the back of my bike and I led the way home.
When we arrived my wife commented that all the cars were really slowing down and giving a wide clearance, presumably because of the balloons attached to my bike

So although not wearing a helmet apparently may cause a driver not to give you any room or space, what seems to be the biggest concern for passing drivers is not how likely they are to hit the cyclist - but whether or not the car will be hit by a balloon!