My dog won't listen to me when chasing a scent or seeing other people/dogs?

My dog is really good and obedient most of the time. I will take her to the park to play and let her off her lead so she can run. She will come when I call unless there are other people around. If someone is walking close by and she starts moving that way she will completely ignore me and run towards them. The same happens when I try to get her to sit before crossing the street. She's only 4 months old. What can I do to make sure that she heeds my call amid distractions?