The pavement, like stone brick, automatically makes a diagonal piece on the inside of 1x1 corners, which enables the player to make smooth roads at eight angles. Could you add a 1x3 diagonal piece to allow sixteen angles? This would make gentle curves look a lot smoother. Alternatively you could add a road-drawing system that works like the rail laying system, you click the green arrow to continue a road, and it can make actual curves. It would be extra neat if it automatically continued whatever pattern you had already placed down!

Example of 1x3 diagonal pieces:
http://i.imgur.com/9K8kzQt.png

I am afraid this is not feasible due to limitations of Factorios tile engine. Only 1x1 tile edges, corners or sides are supported as fair as i know. But i encourage you to post this on the suggestions forum, because this kind of corners would make other types of terrain transitions smoother too.

A road planner? Again a great idea, you're really filling my to-do-list :D
But this sounds more like an idea for a stand allone mod. When i have a little more time, i'll try what's possible.

Sweet! :D