Policy: Objectives & Rationale

Road transport currently accounts for over one-fifth of all UK carbon emissions (Kakkad & Rossiter, 2007). As congestion worsens, congestion charging is viewed as an effective market-based instrument for carbon abatement. One of the central objectives of congestion charging is to cause a mode shift towards public transport and other less carbon intensive modes of transport thereby mitigating aggregate levels of rising vehicle use – partly due to improved highway service levels (Stopher, 2004; Kakkad & Rossiter, 2007). Interest in pricing traffic in an efficient way to reduce externalities negatively impacting health, safety and global warming has increased largely due to environmental awareness and modern information technology making road-pricing systems more viable (Johansson, 1996).

The rationale behind congestion charging is that it may be more effective than other forms of road user charges because it accounts for varying traffic flow conditions. Other emission reducing programs may only regulate the rates (e.g., grams per mile) at which a vehicle emits pollutants but does not account for traffic conditions. For example, a fuel tax is paid proportionately to the use of a road-system without discriminating between traffic flow and capacity (e.g. a rural road at night versus peak traffic in a city centre). Congestion charging however targets driver behaviour with the goal of inducing a mode switch while accounting for temporal and spatial dimensions (Blythe, 2005; Yin and Lawphongpanich, 2006). It is effective provided that it does not simply redistribute traffic flow around a charging zone therefore having marginal impact upon aggregate vehicle-kilometres (Anable and Bristow, 2007).

Another consideration is that congestion charging may not provide incentive to drive a more efficient car, as the charge applied is for the use of road space rather than type of car used. The congestion charge may therefore not necessarily account for the environmental performance of a vehicle within the charging zone which in theory could lead to a situation where a car emitting 250 gCO₂/km is charged the same as one emitting 100 gCO₂/km (Veitch and Bakir, 2007). And although congestion fees are efficient at differentiating between peak and off-peak hours it may have marginal impact upon driver behaviour within the charging zone, notwithstanding increased vehicle speeds due to decreased congestion (Santos and Bhakar, 2005; Veitch and Bakir, 2007). Also, only trips passing the cordon area are charged, while inefficient parking charges and transport prices may persist within the charge zone (Proost and Van Dender, 2001). Congestion charging has also been criticized in that it may only delay reaching certain levels of congestion but will not over the long term reduce congestion or cause a major shift towards public transport. Road pricing on the other hand could be viewed as a better compliment for introducing low emissions and fuel-efficient vehicles (Stopher, 2004). This points to the need for complimentary policies to work in coordination with congestion charging to achieve synergistic affects upon driver behaviour to reduce aggregate carbon emissions.