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The incident took place as the peloton rounded a left-hand bend in the middle of Retford around 50km from the end of stage four of the race from Mansfield to Newark-on-Trent.
With the car parked on the right-hand side of the road around 15m after the corner between Carolgate and Grove Street, on the racing line as the riders swung round the corner at high speed.
The first few riders in the peloton made it safely around the car before Bookwalter appeared to clip the wheel of another, sending him head first into the car’s rear window.
Two other BMC Racing riders, as well as riders from a number of other teams, were caught in the crash, with Bookwalter the only rider suffering injuries as he sat in the road and received medical attention.
Bookwalter abandoned the race and was taken to hospital as precaution, with the rest of the riders able to continue the stage which was won by Quick-Step Floors sprinter Fernando Gaviria.