There are two issues to think about when considering how to name your web pages: SEO and visitor click-through rate. On the SEO side, it is fairly well documented that using keywords in the URL adds slightly to a page’s ranking potential. As far a click through goes, evidence is a bit more vague, but it is generally considered that a spammier looking URL will be clicked on less often than one that looks less spammy–and the more words used in the url, the spammier it will look.
As far as SEO goes, the use of keywords in the URL provides only a very slight ranking advantage and should be limited to the keywords focused on in the optimization of the page. If a page is about household furnishings in general and you are optimizing the page for the keyword “antique bookends” then it would make most sense to use only “camaro” as the page name in that in the URL.
If your page is specifically about the 1969 Corvette and you are optimizing the page for that keyphrase then it is conceivable that the use of all three of those terms would be advantageous for SEO and for click through rate (if someone is search for info on that specific car).
Let’s look at your specific example of “/chevrolet_classic_car_automobile_corvette_1969… vs. “/1969_corvette.html”. At Metapilot, we would see it like this:
1. Your on-page optimization should be focused on fewer words than chevrolet, classic, car, automobile, and corvette. Not only is that just too many broad terms to use for a single page’s optimization, the use of all those terms in the URL is going to dilute any advantage you might get from adding keywords to the URL. Keeping the optimization for a page focused to a single keyword or short keyphrase is often best and the use of a page’s keywords in the URL is just part of properly optimizing a page.
2. It’s just a bit spammy looking and thus, such a URL could have a negative effect on click-through. No one is going to use words combined in that order in everyday speech so besides looking long and weird, it just feels weird, too.
3. When you are using multiple words in your URL, algorithmically it is best to use hyphens rather than underscores to separate your words.
