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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

This report is an archived publication and may contain dated technical, contact, and link information
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Publication Number:  FHWA-HRT-13-045    Date:  October 2013
Publication Number: FHWA-HRT-13-045
Date: October 2013


Cooperative Adaptive Cruise Control: Human Factors Analysis


Because many of the benefits and potential human-factors issues of the CACC concept pertain to shorter following distances between vehicles, it is important to ensure terminology used in this report corresponds to industry standards, especially since the Highway Capacity Manual modified its definitions of gap and headway between the 2000 and 2010 editions.(3,4)

In this report, headway is the time between which identical parts of successive vehicles pass a point on a roadway (e.g., front bumper to front bumper). Gap is the time between which the front bumper of a following vehicle passes the same point on a road as the rear bumper of the preceding vehicle. Many studies researched for this analysis used the term headway when it was apparent they intended the current definition of gap. For example, the procedural section of one study indicated that participants were instructed to estimate "the headway between the front bumper of the vehicle you are driving and the rear bumper of the lead vehicle."(5) Terminology discrepancies such as these have been corrected in this report.

Throughput benefits

The CACC concept purports to improve traffic throughput via two key aspects:(1) decreasing the following distances between vehicles to allow more vehicles to fit in a lane and (2) increasing the flow's string stability, the attenuation of traffic disturbances in the upstream direction, which would reduce traffic jams and increase overall average speed.(6)

The 2010 Highway Capacity Manual indicates the general maximum flow rate for a multilane highway at 60 mi/h is 2,200 vehicles per hour per lane (v/h/l).(4) A microsimulation in which all traffic operated with a 1.1-s gap demonstrated a similar rate, showing a throughput of 2,100 v/h/l.(7) Additional microsimulations have been performed to evaluate the effects of adding vehicles that utilize technologies such as adaptive cruise control (ACC) and CACC.

ACC aids drivers by automatically adjusting longitudinal speed as the immediately preceding vehicle dictates. While this is a convenience for the driver, the available preset time gaps that a driver can select are typically larger than the average time gap seen with manual driving (see Willingness to Utilize Automation section). Therefore, ACC use has been shown to have very little benefit for throughput, especially as the penetration rate increases. (See references 7 - 11.) Typical throughput benefits for ACC peak at about 7 percent over manual driving when penetration is in the 20 - 60 percent range. Beyond 60 percent penetration, there tends to be a negative throughput effect, since more vehicles are traveling at gaps greater than under manual control.(7) Studies have shown conflicting data on the string stability effects of ACC. Some indicate that it helps even out minor fluctuations and disturbances, but others have shown that the time delay of the ACC system regulating speed has a destabilizing effect. (See references 8, 9, 11, and 12.)

Microsimulations evaluating CACC usage have shown the concept to deliver dramatic effects on throughput. Because CACC-equipped vehicles can only utilize a shorter time gap behind other CACC-equipped vehicles, benefits are slow to develop and not evident until the penetration rate approaches 40 percent. (See references 7, 10, 11, and 13.) At that point, however, throughput gains are quadratic and quickly approach 4,250 v/h/l at 100 percent usage when a time gap of 0.5 s is simulated.(7) Another CACC microsimulation allocated several time gaps (1.1, 0.9, 0.7, and 0.6 s) across the simulated CACC traffic and still reflected a highly elevated throughput value of 4,000 v/h/l.(10) However, these substantial results require heavy traffic volumes; at lighter levels, vehicles are already at free-flow rate.

A CACC-specific microsimulation modeled traffic in which non-CACC vehicles were equipped with a "Here I Am" module that broadcasts performance information to allow CACC-equipped vehicles to follow at reduced gaps.(10) While not including the longitudinal control capabilities, this added technology permits CACC benefits to appear at lower penetration rates and rise in a more linear manner. Another study looked at the absolute minimum acceptable gaps to avoid collisions and showed that with CACC, it might be possible to utilize gaps as small as 0.31 s, depending on speed.(14) Furthermore, if CACC technology could react based on the instant brake pressure is applied in the lead vehicle rather than actual vehicle deceleration, this time savings may permit gaps as short as 0.15 s. However, the consequent effect of the much smaller gaps on drivers' capability to steer their vehicles is not yet known.

Because CACC-equipped vehicles perform speed adjustments more quickly than both manual and ACC-driven vehicles, string stability benefits are also realized. Under normal manual control, a small but sudden change in velocity can have an increasing effect upstream, as more and more extreme reactions (later and harder braking) are observed. However, a field study with six CACC-equipped vehicles showed that even with a following gap as short as 0.5 s, string stability was not sacrificed.(6) Stability is not only improved because CACC is able to react quickly to the vehicle immediately in front, but DSRC permits CACC-equipped vehicles to monitor vehicles further downstream and react even before the immediately preceding vehicle has slowed. Some studies have indicated that utilizing distance and speed information for up to three predecessor vehicles in a platoon helps smooth traffic performance.(15,16) Infrastructure-to-vehicle (I2V) broadcasts also promote stability by recommending the same speed for all vehicles (speed harmonization) and by warning drivers or directly influencing a vehicle's speed due to downstream disturbances not yet evident to a driver, including reductions in average speed, accidents, lane closures, or queues of stopped or slowly moving vehicles (queue warning).

Environmental Benefits

The secondary benefit of a more stable, higher-throughput highway environment is a more fuel-efficient state of operation. Fewer traffic jams due to late reactions or overreactions to downstream issues equate to fuel savings. Additionally, I2V communications have the ability to either directly influence a vehicle's performance or inform a driver of downstream issues that could affect traffic flow, such as an accident, road work, or lane merges. By receiving this information before they are visually aware of an issue, drivers have the ability to adjust their speed or position to more efficiently pass through or around the deviation.

In an arterial environment, I2V communications have the ability inform drivers of upcoming intersections and their signal phases, which can smooth deceleration and acceleration rates and reduce the need to come to a complete stop. In addition to simply reducing travel time, these benefits can have a significant environmental effect. A microsimulation of a four-intersection corridor with either manual or CACC-driven vehicles reflected improvements of up to 36 percent in emissions, 37 percent in fuel savings, and 22 percent in average speed for the all-CACC traffic model.(17)