There are far too many instances of drivers following satnav and NOT noticing signs, like width restriction or length restriction.
There is an issue that ordinary satnavs, ie excluding those for trucks or caravans, can and do route drivers down 6'6" wide roads. Ordnance Survey 1:50,000 maps (1.25"/mile) do show narrow roads and most road atlas's exclude them.
Satnavs are very good at knowing exactly where you are, to within 8m but the rest is down to the age and accuracy of the inbuilt map - those who are anoraks and specialise in roads/maps are constantly informing GPS mapping companies of the errors but not all get corrected. Google maps certainly isn't one of the best for accuracy - but it is common and quite user friendly!
HA road direction signs are generally very good, not necessarily the most direct route but almost always the quickest if no congestion.
Satnavs are a good aid, not to be dismissed but never relied on implicitly.