Time Mean Speed Research Articles

Empirical Study of Traffic Features at a Freeway Lane Drop

Traffic was studied upstream and downstream of a bottleneck that arose near a freeway lane drop near London, U.K. using archived high-resolution loop detector data. The bottleneck’s location and mean discharge flows were reproducible from day to day. Further, it is shown that the bottleneck’s discharge flow was about 10% lower than the prevailing flow observed prior to queue formation. Upon bottleneck activation, flow reductions occurring sequentially in time and space marked the passage of the backward-moving shock. Mean shock velocities ranged between 4.8 and 6.4km∕h (3 and 4 mph) as they traveled upstream from the bottleneck. During bottleneck discharge, oscillations arose in the queue and propagated upstream at nearly constant speeds of 17.6–19.2km∕h (11–12 mph). Flows measured at locations downstream of the bottleneck were not affected by these oscillations. These findings were corroborated using data from a freeway lane drop in Minneapolis, Minn. The analysis tools used for this study were curves of cumulative vehicle count, time mean speed and occupancy versus time. These curves were constructed using data from neighboring freeway loop detectors and were transformed in order to provide the measurement resolution necessary to observe the transitions between freely flowing and queued conditions and to identify important traffic features.

Estimating Traffic Stream Space Mean Speed and Reliability from Dual- and Single-Loop Detectors

The relationship between time mean speed and space mean speed that was derived by Wardrop and presented in several textbooks is suitable for estimating time mean speeds from space mean speeds. In most cases, however, estimating space mean speed from time mean speed measurements is desired. Consequently, a new formulation utilizes the variance about the time mean speed as opposed to the variance about the space mean speed for the estimation of space mean speeds. It is demonstrated that space mean speeds are estimated within a margin of error from 0% to 1%. Furthermore, a relationship is developed between the space and time mean speed variances and between the space mean speed and the spatial travel time variance.

Estimating Traffic Stream Space Mean Speed and Reliability from Dual- and Single-Loop Detectors

The relationship between time mean speed and space mean speed that was derived by Wardrop and presented in several textbooks is suitable for estimating time mean speeds from space mean speeds. In most cases, however, estimating space mean speed from time mean speed measurements is desired. Consequently, a new formulation utilizes the variance about the time mean speed as opposed to the variance about the space mean speed for the estimation of space mean speeds. It is demonstrated that space mean speeds are estimated within a margin of error from 0% to 1%. Furthermore, a relationship is developed between the space and time mean speed variances and between the space mean speed and the spatial travel time variance.

A Climatology of Rossby Wave Breaking along the Subtropical Tropopause

Ten years (1986–95) of global analyses from the European Centre for Medium-Range Weather Forecasts are used to investigate the temporal and spatial distributions of Rossby wave breaking (RWB) at 350 K along the tropopause, herein defined by the ±1.5 potential vorticity (PV) unit (10−6 K m2 s−1 kg−1) contours. Though many studies acknowledge RWB as an important contributor to the complex of mixing processes in the atmosphere, there exists no prior climatological study of its distribution near the tropopause. As in previous studies, RWB is identified in the global analyses by southward directed PV gradients. At 350 K, RWB along the tropopause occurs preferentially during summer over the midoceans, in relative proximity to the planetary-scale high pressure systems in the subtropics. Isentropic trajectories at 350 K show that outflow from the tops of these subtropical highs directly participates in RWB over the adjacent, downstream oceanic regions. Two regions are highlighted in this study: the North Pacific during boreal summer and the South Atlantic during austral summer. Synoptic maps of breaking Rossby waves in these regions are provided to reveal the acute tropopause folding in the meridional plane, which characteristically accompanies RWB. The rich interaction between the tropical flow and the extratropical westerly current exhibited by these cases suggests that the subtropical highs serve as important agents in the coupling between the tropical troposphere and the extratropical stratosphere. As expected from theoretical considerations, the locations where RWB occurs most frequently, known as “surf zones,” are shown to coexist with regionally weak time-mean wind speeds and horizontal gradients of PV at 350 K.

A numerical simulation of flow at Fieberling Guyot

A numerical sigma‐coordinate ocean circulation model is used to investigate tidally forced flow near Fieberling Guyot. The aim is to reproduce the observed currents and to identify the dominant physical mechanisms that lead to the complex three‐dimensional flow fields at this tall and steep seamount in the northeast Pacific. Our very high resolution simulation (with 500 m horizontal and less than 20 m vertical grid spacing in the vicinity of the seamount summit) was performed with the newest version of the terrain‐following sigma‐coordinate primitive equation model (SPEM). As in previous more idealized studies, the seamount was placed in the center of a uniformly rotating periodic channel and forced by diurnal barotropic tides. The exponential initial stratification, the tidal forcing amplitude, and its orientation were chosen based on observations from the Fieberling area. Three major characteristics of the flow observed at Fieberling Guyot are reproduced both qualitatively and quantitatively: diurnal currents reach about 20 cm s−1, a twentyfold amplification relative to the tidal flow away from the seamount; the time‐mean anticyclonic flow speeds are close to the observed 10 cm s−1 maximum azimuthal velocity; and the spatial structure of this vortex shows a maximum at about 6 km radius between 400 and 600 m depth, clearly lifted off the bottom. The time‐mean flows are found to be maintained by diurnal waves of a mixed type: the net motion shows characteristics of both bottom‐intensified seamount trapped waves and vertically propagating vortex trapped waves. While the former are mainly responsible for setting up a time‐mean anticyclonic flow along the upper flanks at the bottom, the latter are needed to redistribute the momentum and time‐mean density, thereby reproducing the observed structure with a flow maximum off the bottom. An analysis of the energy, momentum, and density fluxes shows that rectification depends critically on the eddy fluxes that balance the time‐mean downwelling over the seamount summit and upper flanks. The results of sensitivity and parameter studies are utilized to further interpret the role of individual physical mechanisms and the time‐mean dynamical balances.

Evaluation of Speed Measurement and Prediction Techniques for Signalized Arterials

The ability to accurately measure and predict the average speed of traffic for different facilities and conditions has become increasingly important with the implementation of the Environmental Protection Agency's Transportation Conformity Rule. Average speed is a vital input into motor vehicle emission rate models. Three speed measurement techniques (floating cars, loop detectors, and aerial photographs) and two average travel speed prediction techniques (the Highway Capacity Manual method for arterials and TRANSYT-7F for signalized arterials) are evaluated. These techniques are compared and evaluated against 4 hr of field data gathered for an 8-mi (13-km) section of Ventura Boulevard in Los Angeles, California. The mean speeds measured by floating cars and aerial photographs were in close agreement (within 6 percent of each other). Loop detectors generally produced mean speeds 22 percent higher than the floating car measurements, even after correcting for the differences between the space mean speeds measured by floating cars and the time mean speeds measured by the loops. TRANSYT-7F was able to predict the average travel speeds on the arterial within 3 percent of the floating car measurements and 5 percent of the aerial photograph measurements. The Highway Capacity Manual estimates were uniformly 18 percent lower than the measured floating car speeds. A correction to the method improved the estimates but tended to slightly over-compensate. The adjusted estimated speeds were 9 percent higher than the floating car speeds.

A simple stochastic model for freeway traffic

A simple stochastic model for freeway traffic is formulated as follows: each vehicle passing a fixed point in a given lane is assigned as F (follower) or L (leader) with probabilities p, 1 − p. Each car is assigned an independent headway drawn from either a leader or follower distribution. A L vehicle draws its speed from a normal distribution whose mean is the time mean speed. But a F vehicle draws speed from a normal distribution whose mean is equal to the speed of the car ahead of it. Using only two adjustable parameters, the model shows surprisingly good agreement with the statistical properties of freeway traffic over a large range of flow rates as reported in Breiman et al. (1976).

Some Characteristics of Traffic Leaving a Signalized Intersection

Recent studies of a single lane of traffic flow at a site near Enfield, London have led to a simple bunching model that predicts some aspects of the behavior of vehicle platoons leaving a signalized intersection where overtaking is impossible. The relation between time mean speed and flow rate at this site varied considerably with the sampling interval used; possible reasons for this effect are discussed. The experimental work is briefly described and some tentative conclusions are drawn about the nature of the fundamental relations of traffic flow.