Induced traffic from the Sydney Harbour Tunnel and Gore Hill Freeway

Major urban roads have been built without full consideration of the effect of induced traffic attracted by the additional road space. Statistically significant measures, separating out other influences on traffic volume, have been made of the effect of the opening of the Sydney Harbour Tunnel and Gore Hill Freeway, using time-series statistics. The most likely value for short-term induced traffic on the harbour crossings, including the Gladesville Bridge, is 9295 (plus or minus 28%) vehicles per day, while the most likely value of loss of train patronage is 22 141 (plus or minus 42%) passengers per day. The vehicular traffic increase represents 3.4% of the pre-tunnel total average daily traffic in1991 for the harbour crossings, or a 0.157 % increase per additional lane-kilometre of new urban road. No significant change was measured in bus patronage, but a significant drop in ferry patronage was estimated. A large shift from public to private transport therefore probably occurred. The benefits to the induced traffic are, on average, less than the benefits per vehicle to base traffic, while the induced traffic imposes external costs such as congestion, road accidents and pollution. These costs and benefits of induced traffic should be considered in the economic justifications for roadworks.