Multi-objective optimal speed for expressways

Relationships for speed–flow, speed–fuel consumption (speed–carbon emission rates) and speed–accident occurrences for highway (expressway) driving conditions are reviewed. The optimal speeds for maximising traffic flow capacity on highly utilised expressways in Singapore lies in the range of 45–65 km/h, a speed band that the Land Transport Authority (Singapore) uses in its Electronic Road Pricing (ERP). On the other hand, the optimal speed range lies between 50 and 75 km/h based on efficient fuel consumption. As for carbon emission rates of the vehicular flow, the optimal speed range is 50–60 km/h. In terms of accident occurrences, the general norm is that low travel speeds as well as low speed variability are desirable. Experimental runs have shown that a relatively low speed band is preferred for greater fuel efficiency. Setting operating speeds that straddle the optimum regimes of capacity, safety and environmental sustainability suggests a range of 50–60 km/h.