A longer shift means you’re more likely to come up against your 9 or 10 hour driving limit. That is not always displayed on the tacho head, and I’m not sure when the 15 min warning would appear? Would it flash up before 9 hrs if you have a 10 available (but maybe you want to save your 10’s for later in the week). Fortunately I drive a brilliant Actros which replicates the tacho in the dashboard and shows total driving time for the day. (Also for the week, I think the tacho head shows the fortnight)