  We present an empirical phase diagram of the congested traffic flow measured
on a highway section with one effective on-ramp. Through the analysis of local
density-flow relations and global spatial structure of the congested region,
four distinct congested traffic states are identified. These states appear at
different levels of the upstream flux and the on-ramp flux, thereby generating
a phase digram of the congested traffic flow. Observed traffic states are
discussed in connection with recent theoretical analyses.
