Page naming convention - VS _ which is better

I have a new website i am building with several sections. Which is better a - dash or _ underscore in the URLs? And why?

Example:

mydomain.com/blue-work/ford-truck/1999.html OR
mydomain.com/blue_work/ford_truck/1999.html

Both are fine, I always use a "-". Why? Mostly because visually I find it looks better, and also it seems to be more widely used. As far as SE rankings go I have never heard of one making a difference over the other.

Google does NOT treat two_words as being two separate words, and the underscore visibly disappears on underlined links. Avoid the underscore.

Also avoid spaces as they get coverted to %20 by the browser which makes%20the%20URL%20unreadable.

Use hyphens or dots. Those leave the URL readable, and search engines can "see" the separate words as being separate.