Many people are fascinated by cars, boats, and planes. So, there is always a large amount of hobbyists who have taken it upon themselves to model their favorite vehicle in SketchUp. These models end up in the 3D Warehouse, where even more enthusiasts modify them and upload them again. So, you should be OK finding any vehicles you may need. If you need a specific brand and model, you can use one of the commercial collections. As always, a quick Google search will get you started.

One problem with the 3D Warehouse models is that each author has his own style of modeling. If you need 30 different cars for a road scene and each of these has different materials and textures assigned to them, you're going to have a nightmare when you want to change and modify them. To avoid this, build yourself a library of good-quality models with a matching style. Sometimes, these are given away for free by some of the commercial model libraries. Alternatively, spend some time in SketchUp, reducing the...