On the 2014 ride there were two very serious accidents which caused delays to the vast majority of people who were cycling.
I understand through media reports that one gentleman crashed badly into a telegraph pole. A second accident involved a lady injuring her head and / or eye. At least one of those required an air ambulance.
I understand that Twitter has been a bit of a frenzy regarding safety. In my view the bike ride is broadly safe. Cars, although able to use nearly all the roads the cyclists do, can't really move on them due to the volume of people and so avoid the route as a whole.
The Marshall's are also always on hand as well to help, there are 160 odd Marshall checkpoints throughout the ride. They are also marshalling the traffic and making sure cyclists stop at red lights…
There is an official stop point at least every three miles, I actually think a stop point is more frequent than that if you include the unofficial ones.
The biggest issue is the volume of people. We had 28000 people do the ride. When those poor people had their accidents, it caused the snake of cyclists to back up as the Marshall's and police closed the roads. The scheduled gaps that were introduced at the start line evaporated. Everyone ended up being bunched.
Down narrow country lanes this is not good and your awareness of the other cyclists has to be on overdrive.
Slow cyclists you can see ahead and avoid. The random stoppers are less easy to predict but leaving a suitable gap solves that problem, mostly. The one that is much harder to predict is the fast cyclist. Some people are travelling very fast. I travel at about 15-18mph, I had people whizzy past me. Those people normally come up to you and shout to “stay left” so they can get past. They are travelling 25-30 mph or more and you don't get much warning. Keep an eye out for them!
The ride will be as safe as you want it to be. A solo cyclist on a road, practicing for the London to Brighton is less safe than the ride in my view.
If you go steady going down hills, assume that anyone in front of you could stop at any point (aka leave a gap big enough to stop!) and that you keep hydrated and stop if you start to feel ill then the ride is fine and safe.