Possible explanations of phase transitions in highway traffic

It is shown that all the phase transitions in and out of freely flowing tra ffic reported earlier for a German site could be caused by bottlenecks, as are all the transitions observed at two other sites examined here. The evid ence suggests that bottlenecks cause these transitions in a predictable way , and does not suggest that stoppages (jams) appear spontaneously in free f low traffic for no apparent reason. It is also shown that many of the compl icated instability phenomena observed at all locations can be explained qua litatively in terms of a simple Markovian theory specific to traffic that d oes not necssarily include spontaneous transitions into the queued state as a feature. (C) 1999 Elsevier Science Ltd. All rights reserved.