Youngho Kim

RELATIONSHIPS BETWEEN OCCUPANCY AND DENSITY REFLECTING AVERAGE VEHICLE LENGTHS

The linearity of the occupancy-density relationship is investigated by studying the relationship mathematically across a wide range of traffic situations as well as by plotting values of both variables on the occupancy-density diagram. The literature does not show general agreement on the linearity of the relationship. The relationship is shown to be practically linear because the nonlinear factor mathematically derived does not have noticeable effects on the linearity of the relationship. However, the plot of both variables on the occupancy-density diagram shows that the relationship is influenced by varying average vehicle lengths and can be interpreted as being either linear or nonlinear. The proportion of long vehicles (trucks) is tested empirically and is shown to have large effects on the linearity of the relationship.

Innovative Traffic Demand Management Strategy: Expressway Reservation System

Various traffic demand management strategies have been implemented to alleviate traffic congestion, but they have often failed to effectively mitigate congestion in roads, especially when surging demand is concentrated in a short period of time. The proposed expressway reservation system is an innovative and bold strategy in traffic demand management: drivers would make reservations for traveling on expressways. This paper explores the viability of introducing the expressway reservation system during two major national holidays in South Korea. To that end, a survey on the expressway reservation system was conducted. More than 80% of the 1,000 survey respondents said that their perceived travel time during the national holidays was two to three times longer than usual. The percentage of respondents willing to change their departure time to reduce travel time during the national holidays also exceeded 80%. This tendency grew stronger as perceived travel time became longer. Of those who would change their departure time, almost 84% said that changing their departure time up to ±4 h from their initially intended time would be acceptable. Despite such a willingness and flexibility in changing the departure time, only 51.4% of the respondents thought that the expressway reservation system was necessary. Nonetheless, 73.4% of the respondents said that they would participate in the expressway reservation system if it were implemented. Statistical analysis showed that perceived travel time, age, and travel distance were the most influential factors for the publics acceptance of the expressway reservation system.

Multiple-Step Traffic Speed Forecasting Strategy for Winter Freeway Operations

Accurate and timely predictions of traffic conditions are required for congestion avoidance and route guidance in real-time freeway traffic operations. Special attention to winter operations is needed because prediction error could be amplified under severe weather conditions involving snow. This study employed a vehicle detection system to propose a speed prediction methodology that used the k–nearest neighbors algorithm. The speed prediction was further evaluated under different weather conditions with a road weather information system. Cross-comparisons of the mean absolute percentage error (MAPE) between three weather conditions (normal, light snow, and heavy snow) revealed that the MAPE tended to increase with increases in the forecasting time step (T) and snow intensity. The marginal MAPE over the time step was larger during heavy snow conditions than under normal and light snow conditions. These findings indicate that for winter freeway operations, the time step should be selected dynamically, depending on the weather conditions rather than with a static strategy for all conditions. To this end, this study proposes a framework to determine a dynamic forecasting T that is associated with weather conditions.

Systematic Relation of Estimated Travel Speed and Actual Travel Speed

Estimated travel speed based on spot speed measurements from detectors has been used as a proxy to actual travel speed. Recently, actual travel speeds become available as vehicles with onboard electronic devices provide actual section travel times. As a result, two different speeds can be estimated for a freeway section at a time interval: estimated travel speed takes the average of the spot speeds collected from each detector within a section at a time interval, and actual travel speed is estimated using the sampled vehicles’ actual journey speeds over the section. This paper compares the difference between estimated and actual travel speeds using both kinematic wave theory and actual observations for different traffic states. The theoretical analysis proves that there is a systematic relationship between the estimated and actual travel speeds that form a well-defined loop shape. The existence of well-defined relations is verified using real traffic speed data collected from a Korean expressway and a California freeway. Empirical analysis shows that the real data of estimated versus actual travel speeds exhibit a well-defined linear relationship rather than loop shape. The analysis also shows that the estimated travel speed is generally greater than the actual travel speed due not only to the systematic difference between estimated and actual travel speeds but also to the inherent characteristics of speed data from loop detectors. This finding provides a groundwork to calibrate the widely used estimated travel speed by exploiting the actual travel speeds that were provided by a subset of vehicles in traffic.

Traffic model by braking capability and response time

We propose a microscopic traffic model where the update velocity is determined by the deceleration capacity and response time. It is found that there is a class of collisions that cannot be distinguished by simply comparing the stop positions. The model generates the safe, comfortable, and efficient traffic flow in numerical simulations with the reasonable values of the parameters, and this is analytically supported. Our approach provides a new perspective in modeling the traffic-flow safety and the perturbing situations like lane change.

An introduction of indicator variables and their application to the characteristics of congested traffic flow at the merge area

Research on the merge area has mainly dealt with free flow traffic and research on the congested traffic at the merge area is rare. This study investigates the relationship between mainline traffic and on-ramp traffic at three different segments of the merge area. For this purpose, new indicators based on traffic variables such as flow, speed, and density are used. The results show that a negative relationship exists between mainline and on-ramp flow. It is also found that the speed and the density of the right two lanes in the mainline traffic are significantly affected by the on-ramp flow. Based on the correlation analysis of the indicators, it is confirmed that the right two lanes of the freeway mainline are influenced by the ramp flow. The revealed relationships between mainline and on-ramp traffic may help to analyze the capacity of the downstream freeway segment of the merge area in congested traffic.

Reproducibility of Flow-Density Relations in Heavily Congested Flow Based on Individual Vehicle Data

The flow-density relations represent equilibrium relations between flow and density in the stationary state. Using individual vehicle data, this paper extends to heavily congested flow the reproducibility of the flow-density relations found by Cassidy. A new filtering method using standard deviations of traffic variables in a vehicle group built from individual vehicle data is proposed to distinguish the nearly stationary state from the nonstationary state. The traffic data collected from the nearly stationary state with this filtering method show significantly smaller data scattering than do those without filtering in heavily congested flow, even though the data aggregation interval is kept at the same level. This fact indicates that a reproducible flow-density relation also exists in heavily congested flow if the traffic data are collected in the nearly stationary state