Headway Policy Research Articles

Integrated Frequency Allocation and User Assignment in Multimodal Transit Networks

This paper presents an integrated solution method to the frequency-setting problem using optimization and assignment simulation. With a bilevel solution framework, an optimization problem is run at the upper level under the assumption that the total transit demand is fixed. The objective is to maximize wait time savings under the budget, fleet size, vehicle load, and policy headway constraints. At the lower level, an assignment and simulation algorithm is run; the algorithm models the demand response to the new frequency setting and transmits updated ridership and flow values to the upper level. The procedure is repeated until the improvement in the wait time savings converges. The platform is tested on the Chicago Transit Authority network in Illinois for the morning peak. The wait time savings in this experiment are converging and are found to be comparable with the results of a stand-alone frequency-setting algorithm that finds optimal solutions. Results are comparable with the optimal results of a stand-alone platform introduced in another study that modeled the demand response locally by using elasticities as opposed to networkwide modeling using assignment. The integrated platform can be used for medium-term strategic and long-term planning decisions.

Optimal Allocation of Service Frequencies over Transit Network Routes and Time Periods

This study proposes a formulation for the transit network frequency setting problem. The formulation provides an optimal allocation of resources over space and time while recognizing the existence of multiple service patterns along each bus route. Transit agencies must allocate their limited resources optimally to maximize user benefits, operator benefits, or a combination of the two. The coupling of the routes with the service patterns provided along all or portions of the routes is effectively captured, and the user perspective and the operator perspective are merged into one formulation. The service patterns may be scheduled with different subsets of stops for a given route. Users see the resulting combined route headways at the stops. The number of riders varies with the prevailing number of bus trips at a given stop, which is the combination of different pattern dispatch frequencies. Two main formulations are introduced. The first extends work of Furth and Wilson and seeks to maximize the number of riders and the total wait time savings under budget, fleet, policy headway, and bus loading constraints. The second minimizes the net cost under fleet, policy headway, bus loading, minimum ridership, and minimum wait time savings constraints. In both formulations, pattern headways in different time-of-day intervals are the decision variables. This paper provides the mathematical formulation underlying the proposed methodology, describes the solution method and implementation, and demonstrates, by example, important properties of the frequency setting problem in this context, including some that may at first appear counterintuitive.

The adaptive cruise control vehicles in the cellular automata model

This Letter presented a cellular automata model where the adaptive cruise control vehicles are modelled. In this model, the constant time headway policy is adopted. The fundamental diagram is presented. The simulation results are in good agreement with the analytical ones. The mixture of ACC vehicles with manually driven vehicles is investigated. It is shown that with the introduction of ACC vehicles, the jam can be suppressed.

Macroscopic traffic flow propagation stability for adaptive cruise controlled vehicles

Traffic flow propagation stability is concerned about whether a traffic flow perturbation will propagate and form a traffic shockwave. In this paper, we discuss a general approach to the macroscopic traffic flow propagation stability for adaptive cruise controlled (ACC) vehicles. We present a macroscopic model with velocity saturation for traffic flow in which each individual vehicle is controlled by an adaptive cruise control spacing policy. A nonlinear traffic flow stability criterion is investigated using a wavefront expansion technique. Quantitative relationships between traffic flow stability and model parameters (such as traffic flow and speed, etc.) are derived for a generalized ACC traffic flow model. The newly derived stability results are in agreement with previously derived results that were obtained using both microscopic and macroscopic models with a constant time headway (CTH) policy. Moreover, the stability results derived in this paper provide sufficient and necessary conditions for ACC traffic flow stability and can be used to design other ACC spacing policies.

Range Policy of Adaptive Cruise Control Vehicles for Improved Flow Stability and String Stability

A methodology to design the range policy of adaptive cruise control vehicles and its companion servoloop control algorithm is presented in this paper. A nonlinear range policy for improved traffic flow stability and string stability is proposed and its performance is compared against the constant time headway policy, the range policy employed by human drivers, and the Greenshields policy. The proposed range policy is obtained through an optimization procedure with traffic flow and stability constraints. A complementary controller is then designed based on the sliding mode technique. Microscopic simulation results show that stable traffic flow is achieved by the proposed method up to a significantly higher traffic density.

Traffic flow stability induced by constant time headway policy for adaptive cruise control vehicles

This paper is concerned with the traffic flow stability/instability induced by a particular adaptive cruise control (ACC) policy, known as the “constant time headway (CTH) policy”. The control policy is analyzed for a circular highway using three different traffic models, namely a microscopic model, a spatially discrete model, and a spatially continuous model. It is shown that these three different modeling paradigms can result in different traffic stability properties unless the control policy and traffic dynamics are consistently abstracted for the different paradigms. The traffic dynamics will, however, be qualitatively consistent across the three modeling paradigms if a consistent biasing strategy is used to adapt the CTH policy to the various modeling frameworks. The biasing strategy determines whether the feedback quantity for use in the control, is taken colocatedly, downstream or upstream to the vehicle/section/highway location. For ACC vehicles equipped with forward looking sensors, the downstream biasing strategy should be used. In this case, the CTH policy induces exponentially stable traffic flow on a circular highway in all three modeling frameworks. It is also shown that traffic flow stability will be preserved for an open stretch highway if the entry and exit conditions are made to observe the downstream biasing strategy.

THROTTLE AND BRAKE CONTROL SYSTEMS FOR AUTOMATIC VEHICLE FOLLOWING

In this paper, the authors present several throttle and brake control systems for automatic vehicle following. These control systems are designed and tested using a validated nonlinear vehicle model first and then actual vehicles. Each vehicle to be controlled is assumed to be equipped with sensors that, in addition to its own vehicle characteristics, provide measurements of the relative distance and relative speed between itself and the vehicle in front. Vehicle-to- vehicle communication required for the stability of the dynamics of a platoon of vehicles with desired constant intervehicle spacing is avoided. Instead, stability is guaranteed by using a constant time headway policy and designing the control system for the throttle and brake appropriately. The proposed control systems guarantee smooth vehicle following even when the leading vehicle exhibits erratic speed behavior.

THROTTLE AND BRAKE CONTROL SYSTEMS FOR AUTOMATIC VEHICLE FOLLOWING

In this paper, the authors present several throttle and brake control systems for automatic vehicle following. These control systems are designed and tested using a validated nonlinear vehicle model first and then actual vehicles. Each vehicle to be controlled is assumed to be equipped with sensors that, in addition to its own vehicle characteristics, provide measurements of the relative distance and relative speed between itself and the vehicle in front. Vehicle-to- vehicle communication required for the stability of the dynamics of a platoon of vehicles with desired constant intervehicle spacing is avoided. Instead, stability is guaranteed by using a constant time headway policy and designing the control system for the throttle and brake appropriately. The proposed control systems guarantee smooth vehicle following even when the leading vehicle exhibits erratic speed behavior.

A Comparision of Spacing and Headway Control Laws for Automatically Controlled Vehicles1

SUMMARY This paper investigates two different longitudinal control policies for automatically controlled vehicles. One is based on maintaining a constant spacing between the vehicles while the other is based upon maintaining a constant headway (or time) between successive vehicles. To avoid collisions in the platoon, controllers have to be designed to ensure string stability, i.e the spacing errors should not get amplified as they propagate upstream from vehicle to vehicle. A measure of string stability is introduced and a systematic method of designing constant spacing controllers which guarantee string stability is presented. The constant headway policy does not require inter-vehicle communication to assure string stablity. Also, since inter-vehicle communication is not required it can be used in systems with mixed automated-nonautomated vehicles, e.g for AICC (Autonomous Intelligent Cruise Control). It is shown in this paper that for all the autonomous headway control laws, the desired control torques ...

An accident-severity analysis for a uniform-spacing headway policy

Future automatic highway systems should operate at high capacities (≥3600 vehicles/lane/h) over a range of highway speeds (13-26.8 m/s). Under such conditions it would be impossible to eliminate accidents. Here, a methodology to ascertain the severity of one especially critical accident, multivehicle collisions resulting from the emergency braking of a platoon of automatically controlled, closely spaced vehicles, is presented. This includes the specification of a collision model, which was based on reported crash-testing results, the selection of an accident-severity measure, a consideration of a corresponding cost function, and a sensitivity analysis to determine those parameters which most heavily impinge on accident severity and/or cost. The utility of the methodology was demonstrated by applying it to three platoon-accident scenarios which would be especially relevant to automated highway operations, and a quantitiative measure of the effects of key parameters on accident severity is specified. Such results would be of considerable use to a system designer.

Short station ramps for AGT systems

Automated guideway transit (AGT) systems often have off-line stations, so that cars can travel directly from an origin to a destination, bypassing the intervening stations. Trip time is reduced, and a more flexible overall management policy is possible. The disadvantage of off-line stations is that additional guideway is required for the station and its associated ramps. More guideway is expensive, both to build and because of the extra space required. Conventional practice has assumed that the ramps leading into and out of off-line stations should be long enough 1) to accelerate to line speed before entering the main guideway and 2) to leave the guideway before decelerating into the station. Careful analysis using a headway safety computer program shows that ramps usually do not have to be this long. Acceleration and deceleration can safely take place on the main guideway. Possible reductions in ramp length are presented for a range of systems characteristics, assuming a constant headway control policy. These are compared with simple reference solutions. It is concluded that acceleration ramps can usually be eliminated. Deceleration ramps can often be greatly shortened, particularly if main guideway headway is sufficient for successive cars to enter a station.

A headway safety policy for automated highway operations

The maximum capacity, cost, and safety of an automated highway system are largely dependent on the selected headway policy, i.e., the specification of a minimum acceptable headway (as a function of speed) for mainline operations. Here a policy, designed to avert collisions due to "reasonable" lead-car decelerations, is presented and evaluated in the context of achieving high capacity (≥3600 vehicle/lane/hr) over a range of typical highway speeds-13.5 to 30 m/s (30.2 to 67.2 mi/h). This involved a detailed analysis to determine both the relationships between, and the requirements on, the seven parameters which are embedded in this policy. These pertain to systems-level operations, the capabilities of a vehicle's automatic control system, and the vehicle/ roadway interface. The trade-offs associated with safety, capacity, and cost (in the form of required future development efforts) are identified, and three general approaches to selecting parameters for an operational system are specified.