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the ACTUAL risk of incident is still low
Yes, and even allowing for all the other factors, a tester doing an undiminished 35mph will do less damage than a 2500kg SUV (Range Rover Kerb Weight varies from 2454 to 2725 kg) which has automatic safety braked to 15mph before impact.
The issue is not really about the actuarial probability×damage, it's that it only has to happen once for CTT to become practically uninsurable.
Highway authorities aren't imposing a speed limit on pedal cycles, because they can't. CTT course designers, though, should consider whether it's appropriate to send head-down red-mist hands nowhere near the brakes testers at 35mph through a zone where the highway authority thinks the risks merit slowing motor traffic to 20mph