Spotted an article on the BBC website today about sat navs and the variations in the routes that they provide. The article shows that for a particular route (Hertfordshire – Kingsbridge, Devon) there was over 20 miles difference in the length of the suggested routes and over an hours difference in the estimated travel time. The article is pretty basic in its explanation of the variance. The comments provide a better insight into what is going on.
The anonymous “expert” quoted in the article seemed vauge about how routes and times were calculated. The answer is that route finders use maps that have road features that are attributed with information about the speed and any other restriction (width, lanes, direction). By using these attributed datasets, they have a transport network that can calculate route times. Students can get hold of the Ornance Survey’s Mastermap based Integrated Transport Network (ITN) Dataset through the Digimap service.
So if you want to travel from A to B the software looks for the different combinations of routes. They will assume that you travel close to the legal speed limit and factor in delays from roundabouts and traffic lights. This is why you end up geting routed on smaller roads from time to time. Small country lanes may have a speed limit of 60mph but it may be hard to achieve this. The routing software may not take this into account and therefore reports it as the fastest route.
There is often a “cheapest” route suggested. I assume that this looks at routes where you can travel close to 60mph to return the highest efficiency. Routing is complex and can throw up annomolies. I remember using a system a few years ago to travel from Swansea to Edinburgh. The software had me going West, yes, West on the M4 and getting the ferry to Ireland. You then drove through Ireland to catch the Ferry from Belfast to Stranraer. Then on to Edinburgh. The journey was shorter than up the M5 and M6 but it took 2 days. Personally i like technology but also like planning a route myself as i get to look at maps, if i need to buy a new map to do this it is a bonus!