Transport for London has published its latest annual report on the impact of congestion charging.   In the original central London charging zone, congestion further intensified in 2007, and was at the level corresponding to that found in 2002, before the charging scheme was introduced.   This compares with 30% reductions in congestion observed in the early years of the scheme.  The recent worsening is attributed to the removal of road capacity, whether through temporary road works and property developments, or permanent changes such as pedestrian, cycling and bus priority measures.    Consistent with this conclusion is the finding that night-time travel speeds under uncongested conditions have worsened by around 20%.

One factor that is likely to contribute to worsening congestion in the charging zone is ‘induced traffic’.  This has long been recognised to arise when an existing road is widened, or a new road built, and traffic flows generally turn out to be greater than predicted by conventional investment appraisal or scheme modelling.   This extra traffic – the induced traffic – arises because people take the benefits of the investment in the form of additional access to desired destinations, rather than as the saving of travel time as is conventionally assumed.   We can understand this by bearing in mind that average daily travel time has held constant for many years.   Investments to add carriageway lead to immediate higher speeds, but subsequently to extra travel as people take advantage of such higher speeds to journey further in the time they allow themselves for travel.  

A similar effect is to be expected with congestion charging, which displaces drivers who do not wish to pay the charge, leaving more road space for those prepared to pay.   These payers can now access additional destinations by taking advantage of the higher speeds possible.   This extra traffic – induced traffic – tends to undo the congestion relief initially achieved through the congestion charge.   So no great surprise, in my view, that we’re back to square one as far as congestion in central London is concerned.   The main impact of the charging scheme is to generate income to support an improved bus system.