It easy to rely on Sat Nav alone, but with motorists driving into rivers and across fields, they must remember to switch on their brains as well.

WITH 100,000 Sat Navs sold every month it's hardly surprising that, according to a recent poll, satellite navigation systems are now this country's second favourite gadget. Over four million British cars already have a small monitor on their dashboard telling them where to go and this trend for buying hands-free navigation kits is showing no sign of slowing down.

But does our new attachment to stress-free journeys come with an additional price tag? Come the big school holidays, there will no doubt be reports of drivers who were stranded in rivers, caught under bridges or sent to Eastbourne, via Glasgow.

According to Paul Beauchamp, a spokesperson for Ordnance Survey - the organisation which supplies the maps used by satellite navigation systems - drivers need to use their brains as well.

"Sat Navs are obviously a great way from getting from A to B. If you're heading somewhere you've never been before, it can reduce your stress levels and journey time. In addition you'll also end up lowering your fuel bills and reducing congestion, because it will calculate the quickest journey.

"But it's a case of using your common sense.

You shouldn't turn on your Sat Nav and turn off your brain."

Recently, a woman from Somerset drove her car into a brook after following her Sat Nav instructions to the letter. The woman in her 30s drove across a boggy field and down a mud bank.

Beauchamp says this kind of incident isn't uncommon but blames drivers, rather than the technology.

"This is what happens when people become too reliant on their system. It's important to remember that you control the Sat Nav, it doesn't control you. So use your eyes, your brain and road signs.

And, of course, the technology is not infallible; a Sat Nav's signal might go or it could break and then you'd be completely lost. Always take a map as a back up."

But according to Matthew Slocombe, deputy secretary of the Society for the Protection of Ancient Buildings, the increasing use of Sat Nav can cause more lasting problems than a water-logged car.

"The society is very worried by anecdotal evidence that these systems are guiding larger vehicles down unsuitable routes where they pose a threat to historic structures," he explains.

This year we've already seen news reports of cracks appearing on historic bridges, lorries colliding with 300-year-old houses and residents of traditional English villages finding it impossible to cope with the barrage of heavy goods vehicles (HGVs) ploughing through their narrow streets.

"The physical threat to buildings is the real danger,"

says Slocombe, "the danger of lorries bashing into buildings. We hope that manufacturers will improve their systems in recognition of these potential problems."

"The disadvantage of Sat Nav is that while it might take you the quickest way, it's not necessarily be the most appropriate or the best way,"

says Beauchamp. "We map every road but we are not responsible for routing information."

Although there are currently no Sat Nav systems on the market tailored to drivers of HGVs, Beauchamp is optimistic about the future.

"The best thing that could happen is for separate Sat Navs for car drivers and for commercial vehicles - taking into account that lorries are much higher, wider and heavier. Much of this information is available, but it's now up to the commercial sector to say Okay we're going to take this idea and run with it'.

"People ask us why lorries come into their village.

And the answer is that we map everywhere.

If you were in need of an ambulance in the middle of the night, and lived in a small village, I'm sure you'd be very pleased that we map every route in the country."