Design surge pricing for a ride-hailing app with guardrails

A ride-hailing marketplace sees demand spikes at stadium exits and in the rain. At the moment prices are flat and waits go to 20 minutes. Product wants a surge multiplier. Design it: what the multiplier should depend on, how you would estimate how high it needs to go, what it does on both sides of the market, the guardrails, how you would test it, and the failure modes.