The objective of this work is to understand if a relationship between traffic congestion and safetyon urban roadways exist. To do so, crash data over the network of Zurich, Switzerland, is linkedto speed and traffic flow data from the same network. By aggregating the data, the crash risk inrelation to traffic states is analyzed: (i) over the entire network for different times of day; and (ii)for different links on the network for a given time period. In addition, the influence of speed onthe severity of the crashes is also investigated.It is found that at the network level the crash risk (number of crashes per car) is higher duringtimes when the average network speed is lower (5-7 pm). Lower speeds are observed during thistime period typically due to congestion. Hence, at the network level, there is evidence forcongestion being an indicator of increased crash risk. During the same (congested) time period, itis also observed that crashes mostly happened on links with medium speeds, which could be dueto speed variations on individual links being higher during the congested time periods. It is alsofound that, as expected, more severe crashes happen at higher speeds throughout the day.
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