Well given the tag applied, you must have selected the "It's within a (slow) zone" option when answering the quest, which marks speed limits signed by zone entry signs like this https://commons.wikimedia.org/wiki/File:UK_traffic_sign_674.svg. For 30 mph speed limits, if in an unlit area you can select the "sign" option when answering the quest. For a lit area, choose the "no sign, defaults apply" option and answer the following popups.