I generally like programming games like this, and it reminds me a lot of lightbot which is a good thing. I agree with other comments on minor things that could be improved: either give a clearer explanation about how to use the keys instead of mouse to enter commands (especially about the fact that you have to press space to select a track before any keys work, even if there's only one track), or make it so the first track is automatically selected to receive keyboard commands. I'm also not a fan of the kids cheering sound at the end of the levels.

More importantly, this seems like a nice game engine, but it really needs more difficult levels. Maybe looking back at other games like lightbot could help spark ideas on how to make more challenging levels. Or if you changed the mechanics slightly so that bulldozers can push other bulldozers around instead of having the level stop if they collide, maybe that could allow you to design more challenging levels where people need to use that mechanic.