The fact that some Sat Nav companies like Tom Tom have so many devices in use, they are able to report traffic in real-time.[1] It wouldn&#x27;t surprise me if similar averages over a longer period are used to identify speed limits (e.g. What is the mode speed rounded to a multiple of 10, when speeds do not appear constrained by traffic).<p>[1] <a href="https:&#x2F;&#x2F;www.tomtom.com&#x2F;en_gb&#x2F;licensing&#x2F;products&#x2F;traffic&#x2F;real-time-traffic&#x2F;hd-traffic&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.tomtom.com&#x2F;en_gb&#x2F;licensing&#x2F;products&#x2F;traffic&#x2F;real...</a>