Advances in multiagent simulation techniques(1-3) have made possible the st
udy of realistic highway traffic patterns and have allowed theories(3-6) ba
sed on driver behaviour to be tested. Such simulations display various empi
rical features of traffic flows(7), and are used to design traffic controls
that maximize the throughput of vehicles on busy highways. In addition to
its intrinsic economic values, vehicular traffic is of interest because it
may be relevant to social phenomena in which diverse individuals compete wi
th each other under certain constraintsg(9,10). Here we report simulations
of heterogeneous traffic which demonstrate that cooperative, coherent state
s can arise from competitive interactions between vehicles. As the density
of vehicles increases, their interactions cause a transition into a highly
correlated state in which all vehicles move with approximately the same spe
ed, analogous to the motion of a solid block. This state is safer because i
t has a reduced lane-changing rate, and the traffic flow is high and stable
. The coherent state disappears when the vehicle density exceeds a critical
value. We observe the effect also in real Dutch traffic data.