+1 for some indication to confirm the road ahead is indeed flowing ok.
Where I am based, there are some Wazers but not a lot, unless it's peak rush hour you can easily not see any wazers on the map within 20 miles of you. Hence a road can be shown as having no problems simply because no wazer has been there recently.
The old app "Trapster" used to draw a blue line on the roads if another user had used that road within the last hour, that was a great feature as you knew it was clear. But again, so few users, you rarely saw any blue lines / other users!
I would say with the colours, avoid using shades of red/green, there are plenty of red/green colour blind people out there and it could be hard to tell the difference when the road is clear or busy when you're trying to glance at a small screen while driving.