A methodology for modelling and measuring traffic and emission performance of speed control traffic signals

In an attempt to control speeds and reduce crashes, traffic signals are being installed at several locations on highways to serve as speed reduction devices. One concern about this type of signals is that while they may be effective in reducing high speed crashes, they not only stop traffic that is exceeding the speed limit, but other traffic on the approach that is not. As a result, vehicle emissions are likely to increase, because of the existence of excessive delays, queue formation and speed change cycles for approaching traffic. An approach, based on experimental measurements and on modelling traffic and emission performance of speed control traffic signals, is presented here in order to explain the interaction between the signal control variables (for example minimum signal settings, speed threshold setting and minimum green call scenarios) and environmental and traffic performance variables, in particular, carbon monoxide, nitric oxide and hydrocarbons emissions and delay. The experimental data for validation were gathered on Highway N6, connecting the cities of Lisbon and Cascais, in Portugal. The main conclusions of the present research are that, for traffic flow values corresponding to 50% of the road capacity estimate for a traffic signal approach, 85% of the speed violators are effectively stopped, while the fraction of vehicles unfairly stopped reaches 30%. Concerning emissions the presence of signals leads to an increase in CO emissions of about 15%, while NO and HC emissions increase by 10% and 40%, respectively. It was also concluded that the control of speed violators increases with traffic flow. As a trade-off, overall traffic delay will also increase as well as the number of vehicles that are unfairly stopped.