Goshawks don't necessarily see everything ahead on their path, so they must judge the density of the forest and assume they'll find an opening. In an interview with MIT News, aeronautics professor Emilio Frazzoli aptly compares it to backcountry skiing. You don't always know where the next tree stands, but you cruise downhill anyway and assume you'll be able to navigate around it when the time comes. Beyond a certain speed, though, you might not have time to stop or turn before hitting the as-yet-unknown tree. So (if you're smart) you obey an innate, self-imposed, environment-dictated speed limit. Programming this into a robot is difficult, however.