Using microsimulation to estimate highway bridge traffic load

In bridge traffic loading there is an often-made assumption that free-flowing traffic incorporating dynamic effects governs for spans up to about 40 m and that thereafter congested traffic governs. This study uses traffic microsimulation as a basis for the comparison of regularly-used free and congested traffic flow models. Traffic microsimulation offers a comprehensive approach to the modelling of traffic as it models individual vehicle and driver behaviour giving a more realistic picture of traffic states and their consequent load effects on bridges. For a range of bridge lengths and load effects, and for each traffic model, a dynamic ratio that would be required for free-flowing traffic to govern is determined. It is shown that this required dynamic ratio is higher than that expected for a range of spans and load effects. This work concludes that, for a range of spans, the governing form of traffic may be congested traffic, contrary to the common assumption made in bridge traffic load estimation.