Source: http://www.quicksprout.com/2015/04/06/does-url-structure-even-matter-a-data-driven-answer
A common SEO issue often neglected by web masters is poor URL structure. Some say that optimizing your permalinks to be “SEO friendly” is just a waste of time and doesn’t have an impact on rankings. Others would testify that although it doesn’t help much in rankings compared to link building, optimizing it can make it easier for search engines to index your site.
Extraneous characters
– According to Google, it’s best practice to use dashes over underscores because it affects how keywords in the URL are read by search engines. ex. “search-engine” is to “search engine” while “search_engine” is to “searchengine”
Length of URL
– Research shows that search listings are dominated by URLs that contain 35 to 40 characters. Keep in mind that URLs have a very minimal effect on rankings compared to backlinks.
Adding keywords on your URLs
– Avoid stuffing keywords on URLs since Google already downplayed the effect of keywords on URLs as a ranking factor
Subfolders
– From a usability standpoint, it’s recommended to use the least amount of subfolders on your website hierarchy.
In conclusion, backlinks still has a lot of impact with regard to ranking positions. You do not need to focus on URLs as long as they are don’t impact users and site performance.