There have been a few posts on other people's blogs lately which have touched on the issue of litter, fly-tipping etc. on the environment for cyclists.
It may seem like a minor issue, but as the BBC pointed out recently, the presence of litter on the streets is one of those things that makes people feel unsafe.
If people feel unsafe on the streets then they won't cycle. This is what is meant by social safety.
For cycling to be popular the environment has to be non-threatening. This means well maintained streets, well maintained cycle paths, a low crime rate etc. This in addition to making cycling routes direct and making cycling feel like a safe thing to do (subjective safety).
I previously covered the aspect of litter by showing how cyclists here are provided with special litter bins for use while cycling.
Update: "Son of Shaft" posted a comment below with a link to a particularly interesting article in New Scientist about related research. Click here to go directly to the article.