Showing papers on "Headway published in 1976"

Models of Single Lane Time Headway Distributions

Abstract: The movement of traffic past a point is compared to the output of a queuing system having random input. A generalization of the queue output model leads to a suitable headway model: this model is a mixture of two distributions, representing following and nonfollowing headways, in appropriate proportions. Thin model is compared with several others that have been suggested; when used with a lognormal distribution of following headways, it gives the best overall fit to data from the M4 motorway, England, and two-way roads in Indiana, USA. For each site, the parameters of the following headway distribution can be assumed constant. The mean following headways are 1.3 sec and 1.6 sec for the M4 fast and slow lanes respectively, and 2 sec for the Indiana sites. The standard deviation of logarithms of following headways is 0.4 for both M4 lanes and 0.45 for the Indiana sites. For most samples, the reciprocal of the mean interbunch gap λ can be approximated by λ = λ*-1/2 λ*3/2, where λ* is the flow rate, and the proportion of the following vehicles ψ can be approximated by ψ = ρ-1/2ρ-1λ*3/2, where ρ is the traffic intensity.

Relationship of signal design to discharge headway, approach capacity, and delay

Abstract: The rate at which a queue at a signalized approach discharges vehicles has a major effect on the capacity of the approach. To determine the relative effectiveness of various signal configurations and lens sizes in dissipating queues, queue discharge headway measurements were made at 38 sites in five states. Four major classes of signal configurations and two lens sizes were analyzed. The results show that, except for lens size, no class of configurations can be considered better than any other class for any queue position. Estimates are made of expected delay and approach capacity as functions of configuration class and lens size. /Author/

Vehicular headway distributions: testing and results

Abstract: Numerous investigations and studies have been performed concerning the spacings and headways of vehicles in the traffic stream. This paper briefly reviews, examines, and tests some of the mathematical models with real world data collected for both two-lane roadways and urban freeways. These testing methods include both graphical fits and statistical fits, and the results are presented, along with conclusions on the appropriateness of the model representations. The results indicate the composite exponential, Pearson Type III, and log-normal distributions best represent conditions; generally the log-normal distributions are the best for a wide range of traffic volumes.

Analysis and simulation of automated vehicle stations

Abstract: This paper examines station designs and station operating policies which are appropriate to different points along the spectrum of operational headways Stations with multiple off-line ramps or lateral docking are not treated Rather, a specific station design is analyzed for each headway range, and lastly the different types of stations are compared

Longitudinal control and crashworthiness for small automated transit vehicles

Abstract: This study is concerned with longitudinal control and crashworthiness for small, automated transit vehicles. Control system designs and hardware options for relatively short headway operation at moderate speeds are evaluated. Evaluation is based on performance during normal and emergency operations, sensing and communication requirements, and simplicity of design as a measure of cost and reliability. The study is restricted to the vehicle- follower control concept. Controllers are successfully designed for a variety of data sensing and transmission requirements and no problems are encountered in realizing constant safety factor, constant time headway, and constant spacing operational policies. Fundamental analytical results on collision dynamics and the effectiveness of passenger protective devices, shock absorbers, and interior configurations on occupant protection during fore and aft collisions are derived. /UMTA/

Vehicle-follower controls for short headway agt systems - functional analysis and conceptual designs

Abstract: Vehicle following is a technique for the longitudinal control of vehicles in an automated transit system where the speed and spacing of a given vehicle are governed by the behavior of its predecessor on the guideway. Vehicle-follower control at very short headways (0.4 to 3 s) may be achieved by introducing a variable-gain capability. The approach can entail two modes of operation: velocity-command (open-loop) control and regulation (closed-loop) control. Conventional vehicle-follower design is successfully applied in the regulation mode at short headways. However, significant problems can arise when transitions are attempted from velocity-command to regulation mode because of jerk and acceleration constraints for ride-quality considerations. The report investigates the use of variable-gain techniques to resolve the transition problem.

An automatic headway control system based on the use of microwave telemetry link

Abstract: This paper is a study on the use of microwave telemetry links in automatic headway control. It describes a program designed to evaluate harmonic and 2- frequency radars to vehicle mounted radar and to establish the principal limitations and potential performance capability of passive transponders.

Dual mode: automating high cost urban transportation rights- of-way

Abstract: This paper attempts to outline the case for dual-mode transportation systems as the candidate urban transportation system most responsive by far to the entire range of current and foreseeable issues and values governing transportation investments in urban areas. The paper first outlines these issues and describes the resulting criteria that alternately propel and impede the implementation of alternative urban transportation improvements. Analysis reveals that dual-mode transportation systems respond logically and affirmatively to all criteria. Fractional-second headway automated electrically powered guideway systems capable of accommodating a variety of vehicle sizes, and vehicles capable of manual operation off the guideway are indicated. The vehicles would serve both public and private transportation needs through a much larger variety of ownership, pricing, and service schemes than available today. Relative to conventional internal combustion engines, electric power would allow flexibility in the use of basic energy sources, control of air pollution from power generation at relatively few sources, and precise longitudinal control of vehicle location and spacing on the guideway. Condlusions are drawn on the appropriate role and resulting performance requirements for dual-mode transportation systems.

Design of freeway entrance ramps and lane drops by using semi-markov processes (abridgment)

Abstract: A continuous-time semi-Markov highway model can be used to simulate and design highway situations that require a merging manuever. A general continuous-time semi-Markov model can be used to simulate a number of merging situations by the substitution of appropriate values for the design parameters. The substitutions are made into established general formulas. The continuous-time semi-Markov process can then be analyzed with flow graph and Laplace transform techniques. The design parameters included in the model are freeway vehicle headway distributions, lane volumes, lane running speeds, and a gap acceptance function that describes a driver's willingness to accept a given headway in an adjacent lane. The continuous-time semi-Markov model can be used to find the distribution of the time spent by a driver waiting to emerge from an entrance ramp. This distribution can be used to evaluate the freeway entrance ramp designs. The effect of improved visibility on waiting time, for example, could be studied by using the waiting time distribution. Different gap acceptance functions can be used to reflect the effect of improved visibility on a typical driver's merging behavior. The location of a warning that a freeway lane drop is imminent can also be studied by means of a continuous-time semi-Markov model. Design contitions can then be calculated. This proportion can be related to the level-of-service concept. Further work is required before a definitive relation can be established between the output of a semi-Markov model and levels of service as those levels are currently defined.

Design of freeway entrance ramps and lane drops by using semi-markov processes

Abstract: A continuous-time semi-Markov highway model can be used to simulate and design highway situations that require a merging manuever. A general continuous-time semi-Markov model can be used to simulate a number of merging situations by the substitution of appropriate values for the design parameters. The substitutions are made into established general formulas. The continuous-time semi-Markov process can then be analyzed with flow graph and Laplace transform techniques. The design parameters included in the model are freeway vehicle headway distributions, lane volumes, lane running speeds, and a gap acceptance function that describes a driver's willingness to accept a given headway in an adjacent lane. The continuous-time semi-Markov model can be used to find the distribution of the time spent by a driver waiting to emerge from an entrance ramp. This distribution can be used to evaluate the freeway entrance ramp designs. The effect of improved visibility on waiting time, for example, could be studied by using the waiting time distribution. Different gap acceptance functions can be used to reflect the effect of improved visibility on a typical driver's merging behavior. The location of a warning that a freeway lane drop is imminent can also be studied by means of a continuous-time semi-Markov model. Design contitions can then be calculated. This proportion can be related to the level-of-service concept. Further work is required before a definitive relation can be established between the output of a semi-Markov model and levels of service as those levels are currently defined.

Analysis of minimum safe headway for no collisions

Abstract: The paper presents a set of expressions describing the effects of various system characteristics on minimum safe headway. None of the individual relations used in the paper are new to the transit literature; however, two of the concepts should be more heavily emphasized now that automatic transit systems carry an increasing number of passengers. 1) Minimum safe headway should be expressed in terms of the physical interrelations of all pertinent system characteristics, including wind and grade. Similarly, headway criteria should be chosen on the basis of control system capabilities and sufficient failure experience. 2) Differential velocity and differential emergency deceleration must be accounted for in minimum-safe-headway computations. The work presented is not intended to be universally applicable and will doubtless be modified with the application of new safety assurance and control techniques. However, it does represent a rationale for expressing minimum safe headway in terms of specific hardware parameters and environmental conditions. /Author/

Simulation of traffic flow at grade-separated interchanges

Abstract: This paper gives an outline of a traffic simulation model that has been designed to study the effect of the geometric design and traffic composition of a road lay-out on individual drivers and vehicles, and to facilitate the presentation of the consequences of certain designs to those concerned This model is limited to grade-separated intersections with a limited number of on- and off-ramps Magnetovision is employed to give a display of traffic flow on a crt screen The system uses a main program plus 18 sub-routines (in FORTRAN), the work described used an IBM 360/65 computer and required 168 k bytes of memory space Inputs are road and traffic descriptions and outputs are travel time and delay for each vehicle with their means and the acceleration noise of the whole traffic system Factors involved include acceleration/deceleration of vehicles, lateral acceleration, desired speed of the drivers, reaction time, gap acceptance, traffic characteristics, headway distribution based on tanner's m/d/1-queueing model, a car following model, and a simulation process where the model is up-dated according to an incrementation method Validation studies involved an off-ramp, an on-ramp and the approach to a bridge (the roeda sten interchange), all in the gothenberg area Graphs give examples of the distributions obtained by actual measurement and by the simulation model The validation studies although limited, show that the model is likely to be useful /TRRL/

Study of traffic flow on a restricted facility. report iii-1. vehicular platoon parameters: a methodology for traffic control

Abstract: The Baltimore Harbor Tunnel is a restricted facility since the demand often exceeds the Tunnel's capacity. As a result, long vehicular platoons tend to form. This study developed a technique utilizing vehicular platoon parameters for the evaluation of the several control alternatives. Data collected at the Baltimore Harbor Tunnel were used to identify platoons of two or more vehicles utilizing a 200-foot (60.9 m) minimum space headway criterion. The data were also used to determine the number and size of the platoons, the platoon concentrations, the platoon velocities, the average space headways within the platoons, and the number of vehicles within the platoons. Utilizing Greenberg's model of hydrodynamic traffic flow, the platoon concentration and platoon velocity relationship was examined for all the control alternatives. From this relationship, an optimal control policy was developed and an application to on-line computer control was proposed.

An introduction to the definition of vehicle bunching

Abstract: IN A STUDY CARRIED OUT BY THE OECD, A VEHICLE IS REGARDED AS BEING BUNCHED WHEN IT IS TRAVELLING LESS THAN 5 SECONDS BEHIND THE PRECEDING VEHICLE AND THE DIFFERENCE IN SPEED IS LESS THAN 10 KM/H. TO OBTAIN A DEFINITION OF BUNCHING, THE STATE HIGHWAYS DEPARTMENT CARRIED OUT MEASUREMENTS OF THE SPEEDS AND TIME INTERVALS OF ALL VEHICLES ON A TEST CROSS-SECTION OF THE COLOGNE-AACHEN MOTORWAY, USING AUTOMATIC DEVICES. AT THE SAME TIME FIVE OBSERVERS WATCHED TO SEE WHETHER A VEHICLE PASSING THE TEST POINT WAS BUNCHED OR NOT. THE TRAFFIC FLOW ON THE INNER LANE WAS 615 VEHICLES PER HOUR (INCLUDING 35% HEAVY GOODS VEHICLES), AND IN THE OVERTAKING LANE 512 VEHICLES (2% HGV'S). THE AVERAGE SPEED IN THE INNER LANE WAS 95 KM/H, AND IN THE OVERTAKING LANE 114 KM/H. OBSERVATIONS SHOWED THAT ON AVERAGE 14% OF THE VEHICLES ON THE INNER LANE AND 33.7% OF THOSE IN THE OVERTAKING LANE WERE BUNCHED. THE LIMITING VALUE OF THE TIME INTERVAL AT WHICH VEHICLES BEGAN TO BUNCH WAS RATHER LESS THAN 3 SECONDS AND NOT, AS FREQUENTLY SUPPOSED, AROUND 5 SECONDS. FOR THOSE VEHICLES FALLING INTO THE "BUNCHED" CATEGORY, APPROXIMATELY AS MANY POSITIVE AS NEGATIVE DIFFERENCES IN SPEED WERE RECORDED, IN A RANGE VARYING UP TO APPROXIMATELY 50 KM/H. A CHANGE OF SPEED OF 5 KM/H CANNOT BE APPLIED AS A LIMITING CRITERION FOR BUNCHING.

Evaluation of an accelerator position signal

Abstract: To introduce a new rear signal that will frequently light and at the same time often be of no relevance to a following driver is undesirable. It has already been demonstrated that the ability to detect coasting of vehicles can be accomplished by drivers fairly well by using "primary" cues such as the change in the headway gap or visual angle subtended by the vehicle. The only protentially useful information provided by a coasting signal would be to alert drivers to long coasting durations in the event that they are not detected by other means. For this reason, Mortimer suggested that a coasting signal only be presented when coasting durations exceed about 5 s because then the change in speed of the vehicle could be substantial and bring about high relative speeds and changes in headway. The findings of the current study have indicated scarely any benefits attributable to an APS, although they have indicated at least two important undesirable characteristics of it: (a) an increase in the frequency of accelerator releases by following car drivers; and (b) a frequently appearing signal on the rear of a vehicle that on most occasions provides no useful information to the drivers of following vehicles.

Evaluation of an accelerator position signal (abridgment)

Abstract: To introduce a new rear signal that will frequently light and at the same time often be of no relevance to a following driver is undesirable. It has already been demonstrated that the ability to detect coasting of vehicles can be accomplished by drivers fairly well by using "primary" cues such as the change in the headway gap or visual angle subtended by the vehicle. The only protentially useful information provided by a coasting signal would be to alert drivers to long coasting durations in the event that they are not detected by other means. For this reason, Mortimer suggested that a coasting signal only be presented when coasting durations exceed about 5 s because then the change in speed of the vehicle could be substantial and bring about high relative speeds and changes in headway. The findings of the current study have indicated scarely any benefits attributable to an APS, although they have indicated at least two important undesirable characteristics of it: (a) an increase in the frequency of accelerator releases by following car drivers; and (b) a frequently appearing signal on the rear of a vehicle that on most occasions provides no useful information to the drivers of following vehicles.