The Flora London Marathon has four roundabouts along the course. As minor changes to the course occur almost every year, these roundabouts can be run either clockwise or counterclockwise to adjust the overall distance. This allows the start and finish to remain in approximately the same places.
Two roundabouts are located in the Canary Wharf section of the course. Runners pass Canada Square and run along Churchill Place, staying to the left of the median and running around the Cartier Circle roundabout clockwise. As they enter Trafalgar way they move through a gap in the median into the right hand lanes, and run around the Billingsgate roundabout in the counterclockwise direction, returning to the left hand side after the roundabout. Prestons Road roundabout is run clockwise, and runners then have the whole road to a reference point in Poplar High Street.
Measurement of this portion of the course presents safety problems. Most of the way the route is in the left hand lanes, running with traffic. Part of the route is against traffic, between Cartier Circle and Billingsgate roundabout.
Hugh Jones, who measured the course, guided the pre-race validation measurement this year. To keep things safe we rode from Canada Square to Poplar High Street, but did not measure continuously. We measured in the direction of traffic at all times. This allowed us to avoid riding directly into the teeth of traffic on the wrong side of a divided highway. It added a bit of extra distance to be ridden, but preserved a safe ride.
We measured red going out, yellow coming back.