Googling around, it looks like there’s a strong relationship between increased speed and accident severity. The reasons cited are increased kinetic energy of the vehicle, decreased effectiveness of the built-in safety equipment, and a higher risk of rollover. It’s particularly dangerous for pedestrians and cyclists.

Increased speed also increases the risk of an accident, since it reduces the amount of time drivers have to react, and increases the vehicles stopping distance.

From

• https://nacto.org/docs/usdg/relationship_between_speed_risk_fatal_injury_pedestrians_and_car_occupants_richards.pdf and
• https://www.emcins.com/losscontrol/insights-d/2020/08/speed-increases-risk/#:~:text=For every 10 mph of,damage to vehicles and property.

(Interestingly, speeding decreases fuel efficiency, but that isn’t what the original post was about)