Electric cars are just as usable as petrol or diesel ones, or are they?

The Telegraph reports today that users of the Chevrolet Volt and Vauxhall/Opel Apera will have to plan ahead if they are going to be driving up a hill:

'GM revealed its new car will operate in four modes including Mountain Mode, which will need to be activated at least 20 minutes before reaching said mountain so that the petrol engine can be engaged early enough to produce enough electricity to allow full power up hills.'

I may have spotted a flaw, what if I don't know there is a hill coming up ahead?