The 94th edition of the Tour de France will begin with a prologue on the streets of London.
Start & Finish City
Prologue - London > London
BEST VIEWING LOCATION
My current guess is the last left turn which leads onto the finish straight.
If you stand right near before the corner, you can see riders on the outbound and inbound from the same spot. Also, since it is close to the finish, the riders who need to push harder will be taking chances on the final turn.
