Roundabout

By using the Music Box upgrade, people will come running towards you. Use it to make people get near, and then just run them over while letting the Music Box recharge so you can use it more later. 50 kills should come by pretty easily while using the Music Box.

Here's the route i used to get 90 kills, you can also go left at the start and head for the beach which is more open, easier and you're less likely to hit things.

It's very hard without the music box, closest i got was 45 without the box. Use it in small bursts. You'll need 2500 kills to unlock the music box, you can run up and down the beach as the people spawn pretty quick there.