Cooperative Adaptive Cruise Control Human Factors Study: Experiment 2—Merging Behavior

CHAPTER 1. INTRODUCTION

This report describes the second experiment in a series of four that explore cooperative adaptive cruise control (CACC). CACC combines three driver assist systems: (1) conventional cruise control, which automatically maintains the speed a driver has set, (2) adaptive cruise control (ACC), which uses radar or light detection and ranging sensors to automatically maintain a gap the driver has selected between the driver’s vehicle and a slower moving vehicle ahead, and
(3) dedicated short-range communications to transmit and receive data with surrounding vehicles so that the cruise control system can more quickly respond to changes and speed and location of other CACC vehicles, including vehicles that the driver cannot see.⁽¹⁾

When using CACC, drivers share vehicle control with an automated system that includes vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications. Communications between nearby CACC-equipped vehicles will enable automated coordination and adjustment of longitudinal control through throttle and brake activations. Automated control should enable CACC-equipped vehicles to safely travel with smaller gaps between vehicles than drivers could safely manage on their own. Smaller gaps should subsequently increase the roadway capacity without increasing the physical amount of roadway.

The viability of CACC as a successful and widely used technology is dependent on many factors. One of these factors is the ability of drivers to enter and exit streams of CACC platoons. The manner in which drivers enter the CACC platoons relies on the relationship between CACC traffic flow and other non-equipped CACC vehicles. It is believed that, at minimum, in the early stages of CACC market penetration, there will be a dedicated CACC lane. The lanes would function much like high-occupancy vehicle (HOV) or express lanes operate; drivers would only be allowed to travel in the dedicated lane if certain requirements were met. In that case, the drivers would have to be using CACC-equipped vehicles. These lanes would presumably have some sort of physical separation from the regular flow of traffic. This would prevent the disruption of the CACC flow due to non-CACC-equipped vehicles attempting to enter the
traffic stream.

This study is the second in this series of four CACC experiments and explores the ability of drivers to enter and exit dedicated CACC platoon lanes. The goal of this research is to address some of the critical human factors issues for CACC usage related to the abilities and limitations of the drivers using the system. Specifically, the goals of this experiment are to (1) investigate drivers’ abilities to successfully enter a dedicated CACC lane and join an already established vehicle platoon and (2) assess the workload associated with this maneuver.

There are many ways in which CACC can be implemented in the real world, and this study makes several critical assumptions both in terms of vehicle technology and roadway infrastructure. The assumptions made here should not imply that the CACC system will ultimately be implemented in exactly this manner. Rather, they serve as points of reference for addressing potential human factors issues.

The first assumption is based on whether CACC systems will primarily function as V2V or V2I systems. A V2I system implies that platoons will have some external control from a centralized source (or sources). A V2V system, however, implies that CACC platooning vehicles will act in a selfish way. In a CACC system platoon, there are two primary ways in which a driver can enter the platoon in a location other than the front or rear (i.e., the first car or the last vehicle in a platoon). The first way involves requesting permission to move into the platoon (a system that is at least partially reliant on V2I communications). In this case, a driver notes a platoon of CACC vehicles that he or she would like to join. The driver requests permission, permission is granted, a larger gap is provided between two of the platooned vehicles, and the driver is able to enter the platoon. This method adds complexity to the CACC operating system and driver interface.

The second manner in which a driver can enter a CACC platoon is one in which vehicles act selfishly (a primarily V2V reliant system). That is, vehicles will communicate locally with nearby vehicles, and vehicle actions will serve the driver, rather than the drivers nearby. Vehicles will act to maintain personal gaps and speed. This can present an issue when a driver attempts to enter a CACC platoon. Because CACC vehicles will act in a selfish manner, a vehicle will not request entry into a platoon, and an extra gap for that vehicle will not be created. Instead, the driver will merge into the platoon, and the other vehicles in the platoon will adjust speed to restore the desired gap between vehicles and to accommodate the new platoon member. This method results in a variety of human factors issues, especially when vehicles are traveling with short gap distances.

However, a primarily V2V-based system can work from an algorithm that allows vehicles to work cooperatively and selfishly simultaneously. In the case of joining a platoon, the principle vehicle may be able to communicate with already platooning vehicles. The system can then identify the needed speed to enter the platoon, ultimately assuming lateral and longitudinal control of the principle merging vehicle. This is especially important if platoons are travelling with smaller gaps. Shorter gaps between vehicles can increase traffic throughput and ultimately reduce congestion related roadway delays. However, shorter following gaps lead to problematic human factors issues.

At 104.6 km/h (65 mi/h), a 1-s gap leaves approximately 28.96 m (95 ft) between vehicles. Previous studies have shown that drivers feel both comfortable and safe travelling at gaps shorter than 1 s. For example, in an on-road study testing drivers’ choices in following distances, drivers regularly used gap settings shorter than 1 s. In fact, overall, when following another vehicle, drivers elected to set the gap at 0.7 or 0.6 s 8 percent of the time.⁽²⁾ However, with a 0.6-s gap, there is approximately only 17.37 m (57 ft) between vehicles. If an average vehicle length is assumed to be around 6.10 m (20 ft), this leaves less than 5.64 m (18.5 ft) of buffer on either direction for a merging vehicle. As a result, at these shorter distances, drivers may not feel comfortable or have the skill to join the platoon without longitudinal assistance. Similarly, drivers may not feel comfortable allowing the system to assume longitudinal control during a merge. For this reason, driver acceptance of longitudinal acceleration by the CACC system to join a platoon will be explored.

Another very important assumption made in this study is that the CACC system will require dedicated infrastructure in its early implementation. This infrastructure requires that the CACC lane (or lanes) are physically separated from “normal” travel. This is important for several reasons. CACC will be of the most use in congested regions. This congestion often leads to lower travelling speeds and a great deal of speed variation (i.e., stop-and-go or slow-and-go traffic). Because CACC-equipped vehicles travelling in a separate lane will travel at fairly constant speeds with standard gap distances, the lane will be less susceptible to speed variability. As a result, vehicles in the CACC lane are likely to be travelling at speeds greater than the normal travel lanes. The speed differential between the two types of lanes will introduce problems reaching speeds great enough to transfer from one type of lane to the other. Instead, drivers will be required to enter the lane from a separate on ramp (much like drivers entering and exiting dedicated HOV lanes). The physical separation between the two types of lanes also prevents non-CACC-equipped vehicles from entering the CACC lane and disrupting travel
flow stability.

Given these assumptions, there are many human factors issues that arise with merging with CACC platoons. This experiment will explore the following three different types of merges:

• Merging with non-CACC vehicle into a left dedicated lane without CACC platooning and varying vehicle gaps.
  
  
• Merging with CACC vehicle into the middle of a CACC platoon or a continuous stream of vehicles without speed assistance.
  
  
• Merging CACC vehicle into a CACC platoon with longitudinal speed assistance.