Cobra 9-Band Radar Detector
This isn't really a "smartphone accessory" for your car, but it's something millions should have in their car. Cobra has some powerful and useful smartphone apps too, which work with their different model radar or laser detectors.
What we're suggesting here is the best overall value radar detector for 2018. The Cobra ESD7570-9. You'll get 360 degree protection or scanning, tons of features, loud alerts, and useful features to make sure you never get a ticket.