CoastConFan takes a look at driving in the classic era of the Mythos. The embedded videos are a glimpse at just how difficult getting around could be in the early 20th century. As he points out, there's a good reason period drivers carried so many spares.
From a prop perspective, you might find it handy to have a vintage road atlas for any areas you're going to be spending a lot of time in. They can be picked up surprisingly cheaply on Ebay, and you can't beat the level of authenticity they provide. Not to mention how gorgeous classic oil and gas company graphic designs are.
The one drawback to using original source material is that reality can vary considerably from the set-up of a published scenario. One of the segments in Pagan Publishing's "The Walker in the Wastes" takes place near my home in upstate New York. Without giving away any major spoilers, the poor condition of the roads and lack of train access has an impact on the story. In reality, the paved roads and railroad service between the segment location and New York city were among the best in the world in the classic era.