Take a look at London and you'll see it's not only dead-ends that get this treatment. I think there's more going on here… the roads I see showing up less prominently here are the kind that are difficult to drive down (I've been looking around my neighborhood, so speaking from experience)… however they're actually calculating this prominence, it's damned clever.
Edit: in fact, it can even be segments of roads. See https://img.skitch.com/20110104-d3k9f76gttjqbtsg94m85qb3nh.j... (that's all the same street if you zoom in)