They very well may be doing the latter as well, but Tesla has specifically said that their camera reads the speed from road signs[1]<p>[1] <a href="http:&#x2F;&#x2F;www.teslamotors.com&#x2F;blog&#x2F;dual-motor-model-s-and-autopilot" rel="nofollow">http:&#x2F;&#x2F;www.teslamotors.com&#x2F;blog&#x2F;dual-motor-model-s-and-autop...</a>