I have worked on some video soundtracks for clients. You can find tons of free car horn samples on the web. Just launch a search for free car horn sounds or samples. Download the ones you like and just place them in a track where you need them. If you have Kontakt or a sampler that will load the files format like sfz for DimPro you could just load the sample into DimPro and trigger it. I have a vast collection of horns, door slams, cranking up, cutting off, racing by, shifting gears, etc. All sitting in a folder.