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Undertaking or overtaking on the inside refers to the practice of overtaking a slower vehicle on a road using the lane that is kerb side of the vehicle being passed; that is to say, a lane to the left of the vehicle in countries where driving is on the left, or a lane to the right of the vehicle in countries where driving is on the right (see Right- and left-hand traffic). The practice of undertaking, therefore, may only usually occur on a motorway or other road where there is more than one lane in the same direction or when the width of the roads makes this possible (although there may be exceptions in the cases of contraflow bus lanes).
Many countries consider undertaking dangerous and therefore designate it a driving offence, however most countries make the distinction between involuntary undertaking (passing centre side vehicles in heavy traffic) as opposed to the deliberate attempt to pass a slower moving vehicle for one's own benefit.
Legal status by country
- Australia and New Zealand - Undertaking is legal on multi-lane roads, or where a car is indicating to turn right. 
- Canada - Varies by province.
- Finland - Undertaking is specifically prohibited, except for inner-city traffic and vehicle waiting to turn left.
- France - Undertaking is specifically prohibited, except for vehicle waiting to turn left or if the vehicles in the lane to the left are queueing and slow moving.
- Germany - Undertaking is specifically prohibited, exceptions exist for inner-city traffic and overtaking trams and vehicles waiting to turn left.
- Hungary - Undertaking is prohibited outside built-up areas. Inside built-up areas, passing on the right is permitted, even in cases when there are no road markings, but the width of the roads makes this possible. Interestingly, the undertaking manoeuvre in built-up areas are referred as "driving in parallel traffic" instead of "passing on the right" as it is used outside built-up areas.
- Netherlands - Undertaking is specifically prohibited, exceptions include vehicles waiting to turn left, traffic congestion and on roundabouts.
- Poland - Undertaking is legal on 4-lane roads in built-up areas, 6-lane roads outside built-up areas and on one-way roads with marked lanes (this definition includes motorways). (article 24 of Law on Road Traffic) However, similar to the UK it is considered a dangerous practice and is discouraged.
- United Kingdom - The Highway Code discourages undertaking on motorways with some exceptions (rule 268): "Do not overtake on the left or move to a lane on your left to overtake". Undertaking is permitted in congested conditions when frequent lane changing is not recommended. On other roads, the Code advises drivers "should only overtake on the left if the vehicle in front is signalling to turn right" (rule 163). Rule 163 uses advisory wording and "will not, in itself, cause a person to be prosecuted", but may be used in evidence to establishing liability in any court proceedings. On all roads, undertaking is permitted if the vehicles in the lane to the right are queueing and slow moving. Undertaking in an aggressive or reckless manner could be considered Careless Driving or more seriously Dangerous Driving, both of which are legally enforceable offences.[dubious ]
- United States - Undertaking is usually allowed if not expressly forbidden by road signs.