Every time a Google &#x2F; Tesla &#x2F; Ford &#x2F; GM &#x2F; Toyota self-driving car passes down a road in normal conditions, it will be mapping the surroundings in fairly intricate detail.<p>Based on GPS + Lidar + other sensors, the roadway and surroundings will be mapped to within centimeters -- which should make navigating during inclement conditions fairly straight-forward.<p>These systems might fail on snowy roads that have never been traversed previously, but what percentage of driving is performed on roads with hundreds&#x2F;thousands of cars passing every day? 99.9%? 99.99%?