Abstract

This paper describes the behavior of multi-lane freeway traffic, upstream of an oversaturated off-ramp. It is based on empirical evidence from freeway I-880 (northbound) near Oakland, CA. The main findings are: FIFO blockage. Even on wide freeways, an off-ramp queue can grow across all lanes and entrap through vehicles in a first-in-first-out (FIFO) system with similar speeds on all lanes and a well-defined kinematic wave (KW). This can hamper freeway flow much more than an on-ramp bottleneck. (In our case the FIFO regime arose a little over 2 km upstream of the exit and reduced discharge flow approximately to an average of 1500 veh/h per lane across all lanes.) Variable capacity. Under FIFO, the freeway discharge flow can change significantly without a change in the off-ramp flow when the percent of exiting vehicles changes. (In our case, it increased to 5750 veh/h after 30 min at 4520 veh/h when the exit percentage declined from 29% to 24%.) Non-FIFO congested regimes. Multi-pipe traffic states, where queued lanes move at different speeds, can persist for a long time. Presumably, this happens because different drivers prefer different lanes depending on their destination. (In our case, multi-pipe queued regimes were detected immediately upstream and immediately downstream of the FIFO queue.) Semi-congested traffic regimes, where some lanes are queued and others are not, also exist. (They were detected downstream of the FIFO queue.) Regularity of the flow-density scatter-plots. If one excludes the periods of time corresponding to multipipe flow and regime transitions, flow-density points across all lanes are quite close to a "fundamental diagram" with two branches. During multi-pipe periods, flow-density, scatter points cluster along an inner line, parallel to the congested branch, because drivers give themselves longer spacings when driving under these conditions. (C) 2002 Elsevier Science B.V. All rights reserved.