KEY POINTS:
Their common foe is the car, yet Europe's two biggest cities are adopting radically different strategies in their campaign to keep their air clean and their streets free from gridlock.
London has extended its punitive approach to discourage use of the car while Paris has set down a raft of far gentler proposals aimed at weaning car users on to public transport.
On February 19, the area covered by a "congestion charge" in central London almost doubled, extending westwards from the central financial district to cover most of Westminster and upmarket neighbourhoods such as Kensington, Chelsea and Notting Hill.
Drivers who enter the area between 7am and 6pm on weekdays have to pay an £8 ($24) daily fee. Their number plate is scanned by a network of cameras, and drivers who do not pay face fines of up to £150. Residents get a 90 per cent discount on the fee.
The congestion charge was introduced in February 2003, and appeared to have been a revolutionary stroke. Initially set at £5, it caused traffic levels in the zone to fall by nearly 70,000 vehicles per day, or 20 per cent.
The city's transport authority, Transport for London (TfL), added dozens of buses and claims there has been a 72 per cent increase in the number of cyclists on London roads as well as a reduction of around 15 per cent in some of the most harmful types of exhaust pollution.
But are TfL's claims accurate? Last year, cars returned in significant numbers to the central zone: the volume of traffic was only 8 per cent lower than 2003.
Motorists, it seems, have learned to live with the congestion charge. Business people write off the fee against tax, people on flexi-time come to work earlier, before 7am, so they don't have to pay the charge.
Others seem to just shrug, and say it's another part of the cost of running a car.
Ruth Ling, a Labour councillor on the South London borough of Lambeth, admires Mayor Ken Livingstone and his drive against the odds to introduce the congestion charge.
But, she says, "the buses only seem to have got much more crowded and unreliable, and the Tube [underground railway] is still bogged down in problems. The incentive to switch to public transport is frankly very poor".
London's traffic problems have many causes, says Ling. She points the finger at decades of under-investment in public transport and at major demographic changes.
From 2000 to 2005, the population of Greater London rose from 7.3 million to 7.5 million, and large inner-city family houses are now being sub-divided into flats to cope with a rising number of single people, thus encouraging the spread of car ownership.
Paris, meanwhile, has been buoyed by decades of lavish investment in its public transport system, which has given it the relative luxury of using non-coercive options to recover the city centre from the car.
Since March 2001, the creation of bus lanes and tram lines, and tough restrictions on parking, have helped drive car use down by 18 per cent.
Last Friday, the city council, where Socialists and Greens hold the majority, set the goal of reducing traffic levels by a further 22 per cent by 2020, equal to around 800,000 car trips per day.
Whereas TfL is mainly refurbishing its existing train lines, the focus of the Paris transport corporation is expansion.
It will extend a tram network that was launched two years ago; double the number of tracks on a busy section of one Metro line and extend another line; and build an express link between the busy Saint-Lazare and Montparnasse rail stations.
More pedestrian zones will be created, more than 20,000 bikes will be available for rent and public transport will be free for all people who are unemployed or earn below the poverty threshold.
The Ile-de-France region - effectively Greater Paris - has meanwhile announced its own transport blueprint, with a 2030 timeframe.
It calls for extensions to nine Metro lines and three regional express train lines into the deep countryside, and for the constellation of suburbs outside Paris to be interlinked by light railway or tram, saving passengers the need to travel into the city to change trains.
Both London and Paris cite global warming as a virtuous argument for implementing plans bound to be unpopular with car owners.
Livingstone wants a daily charge of £25 for 4WD vehicles, successfully demonised by environmentalists as "Chelsea tractors" for their huge greenhouse-gas emissions.
Paris hopes its plan will cut carbon pollution from city traffic by 60 per cent by 2020.
"No excuse justifies inertia in the face of the threat to the environment," said Mayor Bertrand Delanoe. "Its never easy to change one's way of life. But now it's a matter of urgency."
London
* Mayor: Labourite Ken Livingstone.
* Measures: Congestion fee/fine regime. Extra buses.
* Future: Congestion area has just been doubled. Plans to charge vehicles according to their emissions levels.Highest-polluting cars would pay £25 a day. A third of London's 32 boroughs may introduce different rates for parking permits based on emissions.
Paris
* Mayor: Socialist Bertrand Delanoe.
* Measures: Creation of bus lanes and tram lines. Tough restrictions on parking.
* Future: Further expansion of tram, train networks. More pedestrian zones. Bikes for rent. Public transport free to unemployed, poor.
