Yesterday I did a very informal and unscientific study of helmet use by cyclists in London.

My methodology was simple - I just counted the number of cyclists I saw and kept track of how many of them were wearing helmets. 

Cyclists counted: 48
Total Number wearing helmets: 13
Couriers Wearing Helmets: 0
Helmet Wearing %: 27%

Observations were made between Paddington Station and Soho during the afternoon.  Riders were roughly 3/4 commuters and the rest bike couriers.  I was surprised by the low percentage of people wearing helmets.

In Seattle you still see people commuting and couriers riding without helmets but they are in the minority.  I'd guess they make up maybe 10% of the cyclists I encounter. 

It is extremely rare to see people out training without a helmet in Seattle.  Since I wasn't in a part of London that lends itself to that type of riding due to traffic congestion at that time of day, I don't know how that group of riders would compare to those in Seattle.

I only saw a couple of people with lights on their bikes but it was still daylight.