Accelerating is the main danger for cars, whether that's making them go faster or slower. The benefit of a roundabout is that your acceleration is minimal: you

Accelerating is the main danger for cars, whether that's making them go faster or slower. The benefit of a roundabout is that your acceleration is minimal: you slow down a bit for the turn, and you speed up as you leave it, same as any other curved road. The danger of an intersection is that cars are stopping and starting at it. If a car stops unexpectedly, you have to react. If a car starts moving unexpectedly, you have to account for it. Accelerating to make it through a light before it goes red, or slamming on the brakes in order stop in time for a light, are simply well-known cases of this. This is why highways and freeways are relatively safe: you largely don't accelerate, except at onramps and offramps. Most highway accidents come from a failed overtake: you're accelerating in order to change lanes, and then decelerating to return to the lane. This is also why rubbernecking is dangerous: it's a subtle deceleration that may be missed by the person behind you. It's not actually different for anything else that moves; it's just that other moving things have lower consequences (walking, cycling) or stronger traffic control (trains are on rails; planes listen to air traffic controllers). The closest thing is boats, and boats are rarely both traveling at high speeds and in dense formations.