Freeway Bottlenecks Research Articles

Assessing Freeway Breakdown and Recovery: A Stochastic Model

AbstractConditions at a freeway bottleneck are predicted with a two-dimensional stochastic differential equation (SDE) model of flow and speed. Stochastic behavior is assumed to be standard Brownian motion. To fully describe conditions observed in the field, it is necessary to couple the SDE model with a stochastic capacity model, thus forming a stochastic capacity-differential equation (SCDE) model. The model contains mechanisms to (1) trigger a breakdown event when the traffic flow reaches or exceeds the freeway capacity, and (2) identify when recovery takes place. Measures of freeway performance are derived from SCDE model predictions. They include breakdown probability, expressed as a function of traffic flow and time-of-day, and recovery time, i.e., the length of time the freeway remains in a congested state before returning to a free-flow state. To achieve dependable model forecasts, it is necessary to impose constraints when fitting the stochastic capacity model. The constraint condition consists o...

Revisiting Merge-Influence Area Empirically: Operations Inside Recurrent Freeway Bottlenecks

The current effort aims to analyze traffic operations empirically at merge-influence areas positioned inside recurrent freeway bottlenecks. Four years of data at four bottlenecks in the Milwaukee County, Wisconsin, freeway system were used in the analysis. Bottlenecks were ranked on the basis of merge-area capacity use and were treated individually. The study evaluated how merge-area traffic mix influenced freeway breakdown characteristics such as prebreakdown flow (PBF), queue discharge flow (QDF), queue discharge speed, and congestion duration (CD). Merge-area traffic mix was evaluated with the ramp volume ratio (RVR), which was defined as the percentage of the ramp flow in the total merge-area traffic flow. For bottlenecks with heavily used merge areas, both capacities (PBF and QDF) and queue discharge speed were reduced when RVR increased in value. The value of CD appeared to be sensitive to RVR. The value of CD was found to double, triple, or almost quadruple when ramp flow composed a higher percentage of merge-area flow. Bottlenecks with lightly used merge areas sometimes showed different or even contradictory results. The study also modeled two capacity flows (PBF and QDF) stochastically at merge areas with the three-parametric Weibull distribution. From the current analysis, it appears that the maximum merge-area flow (4,600 passenger cars per hour) recommended by the Highway Capacity Manual is not a conservative design value.

Urban Storage Space Selection Method for Integrated Control on a Freeway Bottleneck

Ramp metering is an effective means to reduce freeway bottleneck delay that results from the capacity drop phenomenon when congestion sets in. The bottleneck flow is kept at free-flow capacity by temporarily storing vehicles traveling toward the bottleneck at the ramp. The metering duration is normally limited because of the finite amount of ramp storage space available to prevent undesired spill back to the urban network. A beneficial extension of the metering duration might be achieved by strategically choosing upstream intersection arms that reduce their inflow to the ramp. For that purpose, coordination needs to be realized between the ramp meter and its intersection controllers located upstream, which will also hinder vehicles not traveling toward the ramp. In this contribution, an evaluation approach is put forward to decide objectively which intersection buffers (arms) should be included in the coordination. To quantify the resulting delays in the system, cumulative inflow and outflow curves are developed as a function of the involved situation-specific variables. This approach enables one to determine the optimal set of coordinated buffers beforehand and to gain insight into the effect of the various system variables on the delays. By means of worked examples, these effects are illustrated, and the way to determine the optimal set of coordinated buffers is shown. Results show that the length of the peak period and the size of the capacity drop strongly determine the coordination benefits, and, hence, a buffer’s minimum required fraction of traffic traveling past the bottleneck needed to result in beneficial coordination.

Microscopic theory of traffic-flow instability governing traffic breakdown at highway bottlenecks: Growing wave of increase in speed in synchronized flow.

We have revealed a growing local speed wave of increase in speed that can randomly occur in synchronized flow (S) at a highway bottleneck. The development of such a traffic flow instability leads to free flow (F) at the bottleneck; therefore, we call this instability an S→F instability. Whereas the S→F instability leads to a local increase in speed (growing acceleration wave), in contrast, the classical traffic flow instability introduced in the 1950s-1960s and incorporated later in a huge number of traffic flow models leads to a growing wave of a local decrease in speed (growing deceleration wave). We have found that the S→F instability can occur only if there is a finite time delay in driver overacceleration. The initial speed disturbance of increase in speed (called "speed peak") that initiates the S→F instability occurs usually at the downstream front of synchronized flow at the bottleneck. There can be many speed peaks with random amplitudes that occur randomly over time. It has been found that the S→F instability exhibits a nucleation nature: Only when a speed peak amplitude is large enough can the S→F instability occur; in contrast, speed peaks of smaller amplitudes cause dissolving speed waves of a local increase in speed (dissolving acceleration waves) in synchronized flow. We have found that the S→F instability governs traffic breakdown-a phase transition from free flow to synchronized flow (F→S transition) at the bottleneck: The nucleation nature of the S→F instability explains the metastability of free flow with respect to an F→S transition at the bottleneck.

The physics of empirical nuclei for spontaneous traffic breakdown in free flow at highway bottlenecks

Based on an empirical study of real field traffic data measured in 1996–2014 through road detectors installed on German freeways, we reveal physical features of empirical nuclei for spontaneous traffic breakdown in free flow at highway bottlenecks. A microscopic stochastic three-phase traffic model of the nucleation of spontaneous traffic breakdown presented in the article explains the empirical findings. It turns out that in the most cases a nucleus for the breakdown occurs through an interaction of one of waves in free flow with an empirical permanent speed disturbance localized at a highway bottleneck. The wave is a localized structure in free flow, in which the total flow rate is larger and the speed averaged across the highway is smaller than outside the wave. The waves in free flow appear due to oscillations in the percentage of slow vehicles; these waves propagate with the average speed of slow vehicles in free flow. Any of the empirical waves exhibits a two-dimensional asymmetric spatiotemporal structure: Wave’s characteristics are different in different highway lanes.

Special issue: emerging technologies for intelligent transportation

Special issue: emerging technologies for intelligent transportation

Variable speed limit control for severe non-recurrent freeway bottlenecks

Variable speed limit (VSL) schemes are developed based on the Kinematic Wave theory to increase discharge rates at severe freeway bottlenecks induced by non-recurrent road events such as incidents or work zones while smoothing speed transition. The main control principle is to restrict upstream demand (in free-flow) progressively to achieve three important objectives: (i) to provide gradual speed transition at the tail of an event-induced queue, (ii) to clear the queue around the bottleneck, and (iii) to discharge traffic at the stable maximum flow that can be sustained at the bottleneck without breakdown. These control objectives are accomplished without imposing overly restrictive speed limits. We further provide remedies for (a) underutilized bottleneck capacity due to underestimated stable maximum flow and (b) a re-emergent queue at the bottleneck due to an overestimated stable maximum flow. We analytically formulate the reductions in total delay in terms of control parameters to provide an insight into the system performance and sensitivity. The results from the parameter analysis suggest that significant delay savings can be realized with the proposed VSL control strategies.

Coordinated Ramp Metering Based on On-Ramp Saturation Time Synchronization

In this paper, a coordinated ramp-metering approach is proposed; the approach is based on the synchronization of the time it takes for an on-ramp to run out of space. The time that congestion and the associated capacity drop can be prevented at a freeway bottleneck by means of ramp metering depends on the available ramp space for temporarily storing the vehicles. For a single on-ramp, the available space and thus the metering time at the bottleneck are often limited. By means of coordination, upstream storage space can be used to prevent a breakdown for a longer period of time. To accomplish that aim, upstream ramps need to reduce their inflow into the mainstream with respect to their own metering task. The extent to which upstream inflow reductions are effectively used to extend the metering time at the bottleneck is determined by the order and timing at which the coordinated ramps run out of space. The proposed approach aims at always saturating the more upstream-located ramps before more downstream-located ones; that is, the ramps run out of space in the downstream direction to make sure that all realized assistance is used by the ramp that is metering at the bottleneck. By means of a simulation test case, the functioning of the algorithm is demonstrated and compared with a no-control scenario, local ramp metering, and an alternative coordination algorithm that aims at filling the ramps equally. Both coordination approaches strongly improve the network performance with respect to local ramp metering, but the proposed coordination approach is able to postpone the breakdown for a longer period of time.

Integrated Variable Speed Limits Control and Ramp Metering for Bottleneck Regions on Freeway

To enhance the efficiency of the existing freeway system and therefore to mitigate traffic congestion and related problems on the freeway mainline lane-drop bottleneck region, the advanced strategy for bottleneck control is essential. This paper proposes a method that integrates variable speed limits and ramp metering for freeway bottleneck region control to relieve the chaos in bottleneck region. To this end, based on the analyses of spatial-temporal patterns of traffic flow, a macroscopic traffic flow model is extended to describe the traffic flow operating characteristic by considering the impacts of variable speed limits in mainstream bottleneck region. In addition, to achieve the goal of balancing the priority of the vehicles on mainline and on-ramp, increasing capacity, and reducing travel delay on bottleneck region, an improved control model, as well as an advanced control strategy that integrates variable speed limits and ramp metering, is developed. The proposed method is tested in simulation for a real freeway infrastructure feed and calibrates real traffic variables. The results demonstrate that the proposed method can substantially improve the traffic flow efficiency of mainline and on-ramp and enhance the quality of traffic flow at the investigated freeway mainline bottleneck.

Variable speed limit control for steady and oscillatory queues at fixed freeway bottlenecks

New variable speed limit (VSL) schemes are developed based on the Kinematic Wave theory to increase freeway bottleneck discharge rates under two scenarios: (i) steady queue and (ii) oscillatory queue. The key principle is to impose VSL control some distance upstream of a bottleneck to starve the inflow to the bottleneck and dissipate the queue. Once the queue near the bottleneck vanishes, another less restrictive VSL is imposed upstream to (i) resolve the heavy queue generated by the first VSL and (ii) regulate the inflow to the bottleneck to sustain the stable maximum bottleneck discharge rate and prevent traffic breakdown. Several strategies are developed for each scenario ranging from the simplest strategy that maximizes the delay saving to more sophisticated strategies for upstream queue management. An analysis of the model parameters suggests that a wide range of the first speed limit (to clear the queue around the bottleneck) can be imposed to realize significant delay savings.

Exploratory analysis of an optimal variable speed control system for a recurrently congested freeway bottleneck

SummaryThis study presents two models for proactive variable speed limit (VSL) control on a recurrently congested freeway segment. The proposed model uses embedded traffic flow relations to predict the evolution of congestion patterns over the projected time horizon, and then computes the time‐varying optimal speed limit to smooth traffic flows. To contend with the uncertainties associated with drivers' responses to VSL control, this study has also proposed an advanced model that further adopts Kalman Filter to enhance the traffic state estimation. Both models have been investigated with two control objectives—travel time minimization and speed variance minimization. Our extensive simulation analysis with a VISSIM simulator, calibrated with field data from our previous VSL field demonstration, has revealed the benefits of the proposed VSL control models. Also, the experimental results indicated that the proposed advanced models with both control objectives can significantly reduce the travel time over the recurrent bottleneck locations. With respect to several selected measure of effectiveness (MOEs), such as average number of stops and average travel time, the research results confirm that the control models with the objective of minimizing speed variance can offer the promising properties for field implementation. Copyright © 2014 John Wiley & Sons, Ltd.

An optimal variable speed limits system to ameliorate traffic safety risk

Active Traffic Management (ATM) systems have been emerging in recent years in the US and Europe. They provide control strategies to improve traffic flow and reduce congestion on freeways. This study investigates the feasibility of utilizing a Variable Speed Limits (VSL) system, one key part of ATM, to improve traffic safety on freeways. A proactive traffic safety improvement VSL control algorithm is proposed. First, an extension of the METANET (METANET: A macroscopic simulation program for motorway networks) traffic flow model is employed to analyze VSL’s impact on traffic flow. Then, a real-time crash risk evaluation model is estimated for the purpose of quantifying crash risk. Finally, optimal VSL control strategies are achieved by employing an optimization technique to minimize the total crash risk along the VSL implementation corridor. Constraints are setup to limit the increase of average travel time and the differences of the posted speed limits temporarily and spatially. This novel VSL control algorithm can proactively reduce crash risk and therefore improve traffic safety. The proposed VSL control algorithm is implemented and tested for a mountainous freeway bottleneck area through the micro-simulation software VISSIM. Safety impacts of the VSL system are quantified as crash risk improvements and speed homogeneity improvements. Moreover, three different driver compliance levels are modeled in VISSIM to monitor the sensitivity of VSL effects on driver compliance. Conclusions demonstrated that the proposed VSL system could improve traffic safety by decreasing crash risk and enhancing speed homogeneity under both the high and moderate compliance levels; while the VSL system fails to significantly enhance traffic safety under the low compliance scenario. Finally, future implementation suggestions of the VSL control strategies and related research topics are also discussed.

Development of control strategy of variable speed limits for improving traffic operations at freeway bottlenecks

A control strategy of variable speed limits (VSL) was developed to reduce the travel time at freeway recurrent bottleneck areas. The proposed control strategy particularly focused on preventing the capacity drop and increasing the discharge flow. A cell transmission model (CTM) was developed to evaluate the effects of the proposed VSL control strategy on the traffic operations. The results show that the total travel time is reduced by 25.5% and the delay is reduced by 56.1%. The average travel speed is increased by 34.3% and the queue length is reduced by 31.0%. The traffic operation is improved by the proposed VSL control strategy. The way to use the proposed VSL control strategy in different types of freeway bottlenecks was also discussed by considering different traffic flow characteristics. It is concluded that the VSL control strategy is effective for merge bottlenecks but is less effective for diverge bottlenecks.

Probabilistic physical characteristics of phase transitions at highway bottlenecks: incommensurability of three-phase and two-phase traffic-flow theories.

Physical features of induced phase transitions in a metastable free flow at an on-ramp bottleneck in three-phase and two-phase cellular automaton (CA) traffic-flow models have been revealed. It turns out that at given flow rates at the bottleneck, to induce a moving jam (F → J transition) in the metastable free flow through the application of a time-limited on-ramp inflow impulse, in both two-phase and three-phase CA models the same critical amplitude of the impulse is required. If a smaller impulse than this critical one is applied, neither F → J transition nor other phase transitions can occur in the two-phase CA model. We have found that in contrast with the two-phase CA model, in the three-phase CA model, if the same smaller impulse is applied, then a phase transition from free flow to synchronized flow (F → S transition) can be induced at the bottleneck. This explains why rather than the F → J transition, in the three-phase theory traffic breakdown at a highway bottleneck is governed by an F → S transition, as observed in real measured traffic data. None of two-phase traffic-flow theories incorporates an F → S transition in a metastable free flow at the bottleneck that is the main feature of the three-phase theory. On the one hand, this shows the incommensurability of three-phase and two-phase traffic-flow theories. On the other hand, this clarifies why none of the two-phase traffic-flow theories can explain the set of fundamental empirical features of traffic breakdown at highway bottlenecks.

Vehicle Reidentification With Self-Adaptive Time Windows for Real-Time Travel Time Estimation

This paper proposes a vehicle reidentification (VRI) system with self-adaptive time windows to estimate the mean travel time for each time period on the freeway under traffic demand and supply uncertainty. To capture the traffic dynamics in real-time application, interperiod adjusting based on the exponential smoothing technique is introduced to define an appropriate time-window constraint for the VRI system. In addition, an intraperiod adjusting technique is also employed to handle the nonpredictable traffic congestion. To further reduce the negative effect caused by the mismatches, a postprocessing technique, including thresholding and stratified sampling, is performed on the travel time data derived from the VRI system. Several representative tests are carried out to evaluate the performance of the proposed VRI against potential changes in traffic conditions, e.g., recurrent traffic congestion, freeway bottlenecks, and traffic incidents. The results show that this method can perform well under traffic demand and supply uncertainty.

Development of a Control Strategy of Variable Speed Limits to Reduce Rear-End Collision Risks Near Freeway Recurrent Bottlenecks

The primary objective of this paper was to develop a control strategy of variable speed limits (VSLs) to reduce rear-end collision risks near freeway recurrent bottlenecks. The risks of rear-end collisions were estimated using a crash risk prediction model that is specifically developed for rear-end collisions in freeway bottleneck areas. The effects of the VSL control strategy were evaluated using a cell transmission model. Several control factors were tested, including the start-up threshold of the collision likelihood, the target speed limit, the speed change rate, and the speed difference between adjacent links. A genetic algorithm was used to optimize critical control factors. For the high demand scenario, the proposed control strategy used 25% of the maximum collision likelihood for the start-up threshold, 35 mi/h for the target speed limit, 10 mi/h per 30 s for the speed change rate, and 10 mi/h for the speed difference between different links. For the moderate demand scenario, the strategy used 20% of the maximum collision likelihood for the start-up threshold, 40 mi/h for the target speed limit, 15 mi/h per 30 s for the speed change rate, and 10 mi/h for the speed difference between different links. The results of comparative analyses suggested that the proposed control strategy outperformed other strategies in reducing the rear-end collision risks near freeway recurrent bottlenecks. With the proposed control strategy, the VSL control reduced the rear-end crash potential by 69.84% for the high demand scenario and by 81.81% for the moderate demand scenario.

Investigations of interactions between bus rapid transit and general traffic flows

SummaryBus rapid transit (BRT) is a popular strategy to increase transit attraction because of its high‐capacity, comfortable service, and fast travel speed with the exclusive right‐of‐way. Various engineering designs of right‐of‐way and the violation enforcement influence interactions between BRT and general traffic flows. An empirical assessment framework is proposed to investigate traffic congestion and lane‐changing patterns at one typical bottleneck along a BRT corridor. The BRT bottleneck consists of bus lane, BRT station, video enforcement zone, and transit signal priority intersection. We analyze oblique cumulative vehicle counts and oblique cumulative lane‐changing maneuvers extracted from videos. The cumulative vehicle counts method widely applied in revealing queueing dynamics at freeway bottlenecks is extended to an urban BRT corridor. In the study site, we assume four lane‐changing patterns, three of which are verified by the empirical measurements. Investigations of interactions between buses and general traffic show that abnormal behaviors (such as lane violations and slow moving of the general traffic) induce 16% reduction in the saturation rate of general traffic and 17% increase in bus travel time. Further observations show that the BRT station and its induced increasing lane‐changing maneuvers increase the downstream queue discharge flows of general traffic. The empirical results also contribute to more efficient strategies of BRT planning and operations, such as alternative enforcement methods, various lane separation types, and optimized traffic operations. Copyright © 2014 John Wiley & Sons, Ltd.

Mainstream Traffic Flow Control at Sags

Sags are freeway sections along which the gradient changes significantly from downward to upward. The capacity of sags is considerably lower than the capacity of normal sections. Consequently, sags are often freeway bottlenecks. Recently, several control measures have been proposed to improve traffic flow efficiency at sags. Those measures generally aim to increase the capacity of the bottleneck, to prevent traffic flow perturbations in nearly saturated conditions, or both. This paper presents an alternative type of measure based on the concept of mainstream traffic flow control. The proposed control measure regulates traffic density at the bottleneck area to keep it below the critical density and hence prevent traffic from breaking down while maximizing outflow. Density is regulated by means of a variable speed limit section that regulates the inflow to the bottleneck. Speed limits are selected on the basis of a feedback control law. The authors evaluate the effectiveness of the proposed control strategy by means of a simple case study by using microscopic traffic simulation. The results show a significant increase in bottleneck outflow, particularly during periods of high demand, which leads to a considerable decrease in total delay. This finding suggests that mainstream traffic flow control strategies that use variable speed limits have the potential to improve substantially the performance of freeway networks containing sags.

Comparison of Estimates of Travel Time Losses on High-Capacity Roads

This paper compares two approaches to estimating travel time losses on freeways that are based on a whole-year analysis. Both approaches are intended to improve the design process for freeways by accounting for the annual traffic demand profile as well as short time variance of demand and capacity. One approach estimates breakdown probability and delay-related time losses by using a probabilistic model. The other approach uses Monte Carlo simulation to estimate travel time losses at freeway bottlenecks on the basis of a queuing model with stochastic demand and capacity. Both approaches are based on annual hourly demand profiles. The approaches are compared for demand profiles and deliver similar but not equal results. Travel time losses for both approaches increase exponentially with traffic volume, with the simulation-based approach consistently generating higher time losses for higher traffic volumes. Both approaches are a substantial improvement on current design procedures, are easy to apply, and are recommended for adoption in traffic analysis guidelines.

Capacity Estimation for Weaving Segments Using a Lane-Changing Model

During peak periods, freeway bottlenecks can be activated by intensive lane changing at weaving segments, where merging and diverging areas are in close proximity. This weaving phenomenon has a major impact on capacity. Much research has been devoted to investigating capacity estimation models for weaving segments. However, because of the model parameters, estimation models are difficult to adopt directly in active traffic management strategies to estimate real-time maximum discharge flow. To this end, this study defined a linear optimization problem to solve weaving capacity and then established a lane-changing model to constrain the weaving flows. The proposed method was evaluated and analyzed for sensitivity with field data from two weaving segments on Whitemud Drive, Edmonton, Alberta, Canada. The capacity estimates from the proposed model were consistent with that from the Highway Capacity Manual 2010 model and with field observations. Moreover, it was also observed that the weaving capacity was sensitive to weaving maneuvers. Finally, the proposed method was applied to estimate the real-time maximum discharge flow rate; the estimates matched field measurements. These findings could lead to implementations in designing optimal traffic control strategies.