London's controversial congestion charge was introduced two years ago amid much scepticism and criticism.

As the biggest scheme of its kind in the world, ensuring the effectiveness of the technology that underpins it was essential.

Transport for London (TfL), the organisation responsible for the capital's transport system, oversees and manages the technology and services contracts behind the congestion charge.

There has been much discussion of the relative pros and cons of the charge, but TfL says the technology infrastructure has been an overwhelming success.

'We wanted to make sure we were using advanced technology, but not necessarily bleeding-edge technology. In other words, it had to work,' said Peter Vine, TfL's technical and commercial head of congestion charging.

'In the case of the number-plate recognition cameras, the technology had been used before. TfL, working with its service providers, basically bolted together components taken off the shelf.

'The glue in the middle, which is the core system, makes congestion charging possible, taking feeds from all the other systems, including the cameras, and that is a bespoke application.'

Capita is providing managed technology services for the congestion charging scheme.

The services company is three years into a five-year, £230m infrastructure contract with TfL to implement, maintain, support and enhance the scheme's technical services, including contact centre management.

Vine says TfL has worked closely with Capita, allowing the two parties to successfully overcome technical challenges and to develop best working practices.

'On day one, the system operated,' said Vine. 'What we did identify was a number of areas where we needed additional computing resources. For example, we realised the need for much larger call centre capacities, although the volume of calls has dropped since its introduction.'

Colin Hooker, IT manager for technology infrastructure at TfL, says there are some challenges with the technology that are still being ironed out.

But the organisation wants to introduce new systems to make payment processes more convenient for motorists.

'We're still looking at improving management information systems used to support the congestion charging scheme and the way we handle vast amounts of data,' said Hooker. 'We want to make the system more automated and auditable.'

Number-plate recognition reporting is difficult, because the system does not automatically update itself if a driver appeals against a penalty charge notice.

TfL is also looking to improve SMS-based payment methods. At the moment, motorists can pay the charge via SMS, but texts must be sent by 10pm on the day the charge is issued.

Subject to approval by the Mayor of London, TfL will extend the facility to allow motorists to text their 'order' until midnight, although a £5 surcharge will be added to their payment.

The organisation is also investigating the possibility of allowing payment from additional credit and debit card types, and lowering the threshold of the congestion charge that applies to fleets of vehicles.

Hooker says TfL is looking to install roadside number-plate recognition technology, to cut network traffic and speed processing times if the western boundary extension of the congestion charge zone is approved.

Mayor Ken Livingstone and TfL are also considering increasing the congestion charge rates from £5 to £8 for ordinary vehicles, with similar increases for TfL's fleet schemes.

• The charge's first two years... The London congestion charge came into effect on 17 February 2003. According to figures from TfL, published in January 2005:
  
  
  

• In the 12 months following the introduction of the charge, the number of accidents in the charging zone dropped by nine per cent during charging hours. This compares with a reduction of seven per cent for the rest of London
  

• 273,000 vehicles, excluding buses and bicycles, entered the charging zone during weekday charging hours in 2004 - identical to 2003 levels
  
  
  

• The rise from £80 to £100 for each penalty charge notice (PCN), introduced in July 2004 - accompanied by images of the vehicle itself on the PCN along with an information leaflet regarding the enforcement process - has contributed to the falling numbers issued each day
  
  
  

• About 200 vehicles are clamped or removed every month. By December 2004, TfL had clamped or removed 1,537 vehicles, recovering more than £850,000 worth of revenue
  
  
  

• 8,000 PCNs were issued in March 2004. In November 2004 only 6,600 were issued.