Take a look at the My Bike Radar ConnectIQ app.

My Bike Radar Traffic
See how many vehicles passed you in a ride. Plus upload your .FIT file after the ride to see exactly where each car passed you. NEW: enable auto-sync through Garmin Connect. You can still manually upload rides to mybiketraffic.com, but you can also...
DCRainmaker has a more detailed write-up at How To: See Map Overlays of Varia Radar Data, with Vehicle Counts | DC Rainmaker