Be honest, depending where you are traveling to & you know roughly how long the journey takes, either by GOOGLR information or the likes, do you say I can afford to go at this time because it takes !!!!!!!! time to get there or do you allow for hold ups etc?
I always allow time for hold ups and the further away I am travelling the more time I allow. The problem often being that I arrive in an area I have never been to before at the crack of dawn and cant find a cafe for breakfast or a cup of coffee to while away the next few hours waiting for my appointed time to arrive.
If it's a route I've not done before I check the approximate mileage and allow approx. 1 hour for 30 miles for a non motorway route, presuming there aren't any major towns to go though. If I'm forced to do a journey mid-late afternoon during school term than I'd abandon all hope of anything like a 30mph average.