Regarding the car sounds...
Judging by some landmarks, and the skybox (both things you can see without noclipping, and things you can see only by noclipping), allow you to place the entrance and exit of the hazard course in relation to eachother. From the exit, where you are riding, you can see a car pulled over on a road about a half mile or so ahead. If you look to the left of it, you will see a set of towers identical to the ones from the intro to the mod. That road runs right next to the cliff that drops into the area you started in.
See these two images. So whoever was in charge of that level (or the skybox, or whatever) probably had that road in mind as the source.
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madcat1030
Mikel joined
Hi, the name you shall know me by is Mikel T Strawenaskevitz. I consider myself an average MOD-maker (with honors in story-development), and currently, my only work that is going to stay in progress is a (hopefully) full-length Portal mod. Thanks for checking out my profile, and here's to good modding and mod-seeking!