Traffic Signal Cycle Research Articles

Discrete release rate impact on ramp metering performance

Ramp metering, if properly applied, is a direct and efficient means to avoid or reduce the space-time extent of motorway congestion and sensibly to improve the merging conditions. Regardless of the ramp metering algorithm employed, the metering signals may be operated in various ways based on the ramp metering policy adopted. Ramp metering policies include traffic signal cycle, 2- or n-cars per green, and discrete release rates. In the latter policy, a number of discrete release rates are prespecified, each implemented with a specific cycle and green phase. This approach allows for short green phases (small platoon releases) whenever possible but also for high ramp flows when necessary. We address the problem of determining the lowest required number of release rates that will not affect ramp metering operation compared with the theoretical case of any (even decimal) release rate. Results from investigations using the ALINEA (Asservissement Lineaire d'Entree Autoroutiere) ramp metering algorithm and the METANET (Modele d'Ecoulement du Trafic Autoroutier: NETwork) macroscopic traffic simulator are reported and discussed in detail. Finally, recommendations are provided concerning the lowest required number of release rates and the discretisation scheme to be used. (A)

Measurements of roadside air pollution and traffic simulations

Air pollution levels at the roadside were measured and trallic flows were analyzed in order to determine the dynamic features of air pollutants. Moreover, a t.raflic simulator was used to simulate observed tratTic flows and to evaluate proposed strategies. It was found that temperature-inversion above the road surface due to vehicle exhaust emissions might suppress the vertical dispersion of air pollutants in the early morning. It was also found that concentrations of air pollutants at the roadside near traffic lights had periodicity corresponding to the traffic signal cycle. But the phase of nitrogen dioxide concentration was different fkom that of suspended particulate matter. It was found that total traffic volume depended mainly on the number of small car, but level of air pollution along the roadside seemed to depend on mainly on the number of heavy-goods vehicles, A vehicle-actuated signal-control system that enables reduetion of the air pollution level at the roadside is proposed.

Queueing Algorithm for Calculating Idling Emissions in FLINT—the FLorida INTersection Air Quality Model

The theoretical development of the queueing model used in the FLINT (FLorida INTersection) air quality model is described. FLINT is an area source model used to predict carbon monoxide concentrations for under-saturated and oversaturated traffic conditions at signalized intersections. In the FLINT model, deterministic queueing is used to estimate the queue length for cases of undersaturated conditions. In oversaturated cases, a cycle failure method has been developed to estimate queue length. In addition, a new concept is presented for calculating idling time for each vehicle’s position in the queue during both the red and the green phases of the traffic signal cycle. A limited set of undersaturated cases from monitoring data in Melrose Park, Illinois, was used to compare the predicted queue lengths with the measured queue lengths for several air quality models. It was found that FLINT predicted the queue length within one vehicle of the observed queue length. The same cases were tested using CAL3QHC, TEXIN2 intersection air quality models, and the American Automobile Manufacturers Association (AAMA) simulation model. It was found that predictions of the AAMA and the FLINT models were very close to the measured queue lengths in cases of undersaturated conditions. Moreover, although the FLINT and the AAMA models use a different approach to estimate queue length, their predicted queue lengths were very close in oversaturated cases. However, the predicted queue lengths of CAL3QHC were too long for oversaturated cases.

Incident Detection on an Arterial Roadway

Presented here is the development of an automatic incident detection algorithm for use on Lakeshore Boulevard, Toronto, Canada, based on volume or occupancy data recorded from fixed-loop detectors. Four prospective logics were based on 20-sec intervals; the remaining five were based on traffic-signal cycle lengths to eliminate the fluctuations in 20-sec data. To identify the detection ability of each logic, data from a known severe incident (a two-lane blockage) were used. Only one logic exhibited promising results from the initial development and feasibility test: the logic that compared current cycle volume and occupancy values with those averaged over the previous 3-, 5-, and 10-cycle periods. Further evaluation was conducted on this logic. A data base was developed around two additional reported serious incidents; the logic detected both incidents before their official start times. A second data base, which consisted of data from 13 days, was developed to test the overall performance of this logic, focusing specifically on false alarms. On average, one unexplainable false alarm was reported for every 5 hr of data tested for the entire Lakeshore system (49 detector stations). Testing with additional Lakeshore incident data in a real-time environment is required to fully investigate the ability of this algorithm to detect serious incidents. As developed, this logic cannot distinguish between congestion due to incidents and recurrent congestion. The latter was responsible for the majority of the alarms as tested and thus future improvements should focus on this aspect.

Driver's decision process on termination of the green light

The paper describes the background and evaluates the effectiveness of a flashing green phase just before the amber leading to red phase in the traffic signal cycle. This type of signal sequence, practiced in Israel and a few other countries, was thought to provide drivers with useful information which should lead to better stop or go decisions. However, accident analysis studies have shown an increase in collisions following the introduction of a flashing green. A laboratory simulation study designed to explain the accident findings was conducted. It was found that the flashing green elicited from the drivers an earlier decision response and a higher number of inappropriate stopping decisions, particularly close to the intersection. The pattern of stopping and crossing decisions in the flashing green program suggests more friction between stopping and nonstopping vehicles and, therefore, a higher likelihood of front-rear collisions compared to a regular program.

The Development and Evaluation of Metropolitan Toronto's Real Time Program for Computerized Traffic Control Devices

Under the tri-level sponsorship of the Canadian Federal Transportation Development Agency, the Ontario Ministry of Transportation and Communications and Metropolitan Toronto, the staff of the Metropolitan Toronto Roads and Traffic Department developed a computerized traffic signal control program capable of optimizing and implementing on line traffic signal splits, cycle lengths and offsets within 15 minute control periods. Prior to a detailed field evaluation, the various real time program components were subjected to sensitivity testing in order to ascertain their practicability within a real world environment. During the Fall of 1975 the on line and an optimized set of fixed time/time of day signal settings were evaluated in a suburban and a central area signal network within Metropolitan Toronto by collecting link traffic volume, travel time and delay data. This data served as the base for a series of comprehensive statistical analysis oriented towards an evaluation of network travel time and service rate and a link by link paired comparison.This statistical analysis indicated that the real time optimization program settings outperformed the fixed time time of day settings by 3.6 percent in the central area; whereas in the suburban area, the fixed time time of day settings were superior by 6.3 percent. Overall a number of areas in the real time program were identified for improvement; but more important the role of on-line responsive signal timing packages has been demonstrated for computerized traffic control applications.

Minimizing delay at a signalized intersection for time-invariant demand rates

A method is described for apportioning the green time of a traffic signal cycle at an intersection among a set of stages (phases) in such a way as to minimize the total or average delay to all the vehicles which use the intersection. It is a variation of the linear programming methods of Allsop and Yagar which maximize capacity for specified upper limits of saturation on the approaches subject to arrival rates being in a specified ratio. By stating the cost functions in a different form it is possible to minimize the total delay, allowing any levels of saturation that are justified in the total system sense. Throughflow equivalents are used for turning movements to allow for lane-sharing by the through and turning vehicles. These flow equivalents can be effectively varied as required from one stage of the traffic signal cycle to another to accommodate variable effects of turning movements on capacity and delay, such as changing the mode of operation for left-turning vehicles from absolute right-of-way to a search for appropriate gaps in priority streams. The solution procedure requires that the cost function at each approach be stated in terms of delay vs effective red time for the known arrival distribution. Approximations are introduced through the use of turning equivalents where certain movements must necessarily share road space and through piecewise-linear approximations to the above delay functions. The arrival distributions are assumed known and stationary. An example of the total method is illustrated for a hypothetical 4-way intersection.

Capacity of a signalized road junction: Critique and extensions

Earlier methods for maximizing capacity at a signalized at-grade road junction are examined and compared. Allsop's linear programming method is considered in detail and its shortcomings discussed. His basic method is reformulated and extended to increase the scope of application. The proposed method is able to treat intersections where saturation flows may vary from one stage of the traffic signal cycle to another even though vehicles from certain approaches may legally move in both stages. Such variations in saturation flow might arise, for example due to vehicles in a certain approach having absolute right-of-way in one stage but needing to find gaps in a priority stream during another stage. Some attention is given to mixed movements in a single lane. Also, the effective lost time is treated in a manner based on Webster's method. End-of-stage bulk turning movements are treated in terms of saturation rates by allowing the possibility of negative effective lost times. Examples identical to Allsop's are treated by the proposed method except that some limited gap-seeking is allowed. The increase in predicted capacity ranges only from 0 to 2 per cent in the particular examples used, the proposed method yielding the higher estimate due to its ability to represent gap-seeking as well as ultimate right-of-way. The flows on the various approaches are constrained to be in a given ratio.