There are no specific rules for sat nav and mobile phone placement. Instead, drivers are under a general obligation to keep a clear view of the road. The law isn't very clear, so there's not really a correct place to mount a sat nav as such — however, drivers and police are expected to use their common sense when it comes to a device obstructing the driver's view of the road. In fact, the situation is such a grey area that Greater Manchester Police was heavily criticised a few years ago after tweeting that “everywhere else is illegal” last year alongside a photo of a sat nav in the bottom right-hand corner of the windscreen, despite the law making no mention of where to legally place a sat nav.

The bottom right-hand corner is a sensible placement (you'll see a lot of taxi drivers do this, too), but whether you'd be stopped would be entirely up to that particular police officer's interpretation of the law and whether your view was obstructed by the sat nav. If we consider the legal placement in terms of what would pass or fail an MoT — splitting the windscreen into zones A and B — Zone A is said to be a 290mm wide strip above the centre of the steering wheel, up to the highest point the windscreen wipers reach, while zone B is the rest of the windscreen. Obstructions should not “encroach more than 10mm” within zone A or “encroach more than 40mm” in zone B. It may be impossible to fit a sat nav to the windscreen without breaking these rules because very few devices measure under 40mm across. However, MoT testing criteria also states that if an “obstruction does not impair the driver’s view of the road, the vehicle should pass. If it only affects the driver’s view of the sky or the bonnet then this is not to be considered a reason for rejection”. This suggests it may be allowed to position a device measuring more than 40mm/10mm in the windscreen if it's high enough or low enough to only obscure the dashboard or sky, without affecting your view of the road ahead.

There are specialist holders on sale for sat-navs and phones, which can be fixed into air vents to prevent the windscreen from being obstructed. For those who have to place it on the windscreen, low down to the right or left-hand side out of view is safer.