I believe the CSS report can still be purchased, we bought one a few years ago prior to LTN 1/09.

The part time roundabouts sampled in the CSS report showed a 66% increase in accidents when the signals were off. As all other factors remained constant the conclusion was it was the part time effect which leads to the accident problem. I can let you have some more information from the report regarding the assumptions made if you want to contact me – carole.probert@amey.co.uk.