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Coordinating, Developing, and Delivering Highway Transportation Innovations

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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


As previously discussed, the CACC concept has several benefits. Closer following distances and improved string stability of traffic flow reduce traffic jams and increase overall throughput. Additionally, these effects produce secondary benefits, including reduced fuel consumption and emissions. In a large urban freeway environment, the impact can be substantial. In arterial settings, large or small, CACC supplemented with I2V communications can produce similarly effective results. Implementing the concept in these two environments, however, poses challenges that need to be considered.

Freeway Evironments

The greatest impact of traffic congestion is on large urban highways, so it makes sense to focus on promoting the CACC concept in these environments. However, a CACC-equipped vehicle is not able to travel at reduced time gaps unless the preceding vehicle is also equipped and has the system actively engaged. This means that until the penetration rate reaches a certain level, around 40 percent based on several microsimulation studies, throughput improvements will remain elusive. (See references 7, 10, 11, and 13.) Getting this new technology into a sufficient number of vehicles is difficult for several reasons.

First, vehicles in the United States are being kept for longer periods, slowing the introduction of new cars equipped with the latest technologies. From 1995 to 2009, the average age of U.S. light vehicles (passenger cars and light trucks) increased 21 percent to 10.2 years.(18) The factors for this rise include an increase in quality and durability of vehicles and economic issues that likely encourage longer ownership.

Second, car manufacturers are reluctant to introduce technology (and cost) to a vehicle if it is not seen as an immediate benefit to the consumer. If a driver is unable to utilize CACC due to low penetration in surrounding vehicles, he is unlikely to spend extra money for the technology. Many new technologies are rolled out slowly, first in luxury vehicles in which cost is not typically a major factor for the consumer, then trickling down to economy vehicles as production costs decrease and benefits increase. Even at the point at which CACC could be included in all new cars, long average turnover rates imply an extended rollout period.

Third, retrofitting technology into existing vehicles is not always easy or cost-effective. Standards adopted to implement a technology in new vehicles may not be viable for older vehicles. Additionally, the aftermarket cost of a technology is usually much higher than original equipment, decreasing the likelihood of adoption.

Therefore, implementing the CACC concept in a freeway environment will likely require a staged approach.

Restricted/Managed Lanes

Similar to the use of restricted lanes to encourage carpooling or alternative-energy vehicle usage, restricted lanes could be utilized to congregate CACC-equipped vehicles and increase penetration rates in those lanes. Although the overall penetration rate would be low, the rate in restricted lanes could rise enough to demonstrate throughput increases. Limiting the initial infrastructure needs to provide I2V communication to a few managed lanes would also allow capabilities to grow as needed and keep initial costs at a minimum. Although I2V communications need to be thoroughly vetted before being introduced, a staged approach to communicating with a larger and larger set of vehicles could help ensure a better operating environment.

Restricting CACC usage to a lane or set of lanes involves hurdles and drawbacks, however. The locations that could benefit most from traffic relief, urban areas, are also more likely to have limits on freeway expansion. If restricted lanes are not already utilized and new lane construction is not possible, reserving one or more existing lanes for a small percentage of vehicles would restrict non-equipped vehicles to fewer lanes, worsening overall traffic flow.

Areas that already utilize restricted lanes (e.g., for high-occupancy vehicles (HOVs)) could permit CACC-equipped vehicles to join those lanes; however, the magnitude of the effect may depend on the number of restricted lanes and the other vehicles permitted to use them. In scenarios with a single restricted lane, CACC-equipped vehicles would be blended with non-equipped vehicles, increasing volume but eliminating CACC benefits except in the case of coincidental platoons. Additionally, if the restricted lane was intended for HOVs, permitting single-occupant CACC-equipped vehicles to join (with little benefit) may reduce HOV benefits, reducing throughput. In order for a symbiotic relationship to succeed with mixed equipage vehicles, extensive education of and cooperation from lane users may be required. V2I communication could help CACC-equipped drivers but would not be useful for non-equipped HOV drivers.

However, with two restricted lanes, lane usage could be split to consolidate CACC-equipped vehicles. The left lane could be restricted to CACC-equipped vehicles and the right lane to CACC, HOV, and alternative-energy vehicles. Dual lanes could also decrease the possibility of controlled lane users from being restricted to a single lead-vehicle speed. As previously stated, however, dual restricted lanes may not be possible in many areas most in need of CACC technology benefits due to space or budget.

With any use of restricted lanes, automated controls would be necessary to prevent non-equipped vehicles from utilizing the lanes. Unless CACC-equipped vehicles include an externally visible indicator when the system is engaged, CACC usage cannot be visually enforced as can be done with HOV restrictions. However, DSRC broadcasts by equipped vehicles could be used to grant access if restricted lanes are physically separated. If the lanes are not physically controlled, checkpoints could identify vehicles not broadcasting and permit photo enforcement, similar to current speed and red-light camera technologies.

Once CACC penetration rates increase to a near majority, it is less likely that an area would need specialized lanes to obtain CACC benefits. The restricted lanes could simply join the overall freeway pool (unless a region maintains a need for HOV or alternative energy vehicles), the infrastructure for I2V communications could be expanded to cover most or all lanes, and throughput increases would be more widely realized.

Full-Lane Coverage

Phased implementation would allow drivers of non-equipped vehicles to witness the benefits, providing demand for the technology. This, in turn, would give car manufacturers the incentive to provide the technology in more vehicles, and ultimately, CACC would become a commonplace technology, similar to CCC.

As Su's 2011 microsimulation indicated, adding a "Here I Am" module to non-CACC vehicles would permit CACC-equipped vehicles to follow at shorter gaps.(10) This may provide enough artificial penetration to achieve benefits earlier than projected and permit all freeway lanes to experience throughput increases. Although this would involve retrofitting existing vehicles, it would not involve the more extensive requirements to regulate the vehicle's throttle and brake activations and, therefore, would likely be an easier, less expensive solution. Policy related to how to encourage or pay for this retrofit would need to be researched.


The arterial environment is more dynamic than a typical freeway, including intersections, a wide variety of vehicle maneuvers, commercial and residential driveways, and pedestrians. Therefore, rather than providing a shorter time gap, CACC has the potential to produce better string stability and a reduction in delays and stops. Benefits of V2I and I2V communications could include more efficient movement of vehicles through an arterial section, saving travel time and reducing fuel usage and emissions.

As a CACC-equipped vehicle approaches a red-light intersection, SPAT information could be utilized by the infrastructure to determine the most appropriate speed for the vehicle to pass through as quickly as possible without stopping. The I2V transmission could automatically adjust the vehicle's speed or simply provide the suggested speed to the driver.

In the opposite manner, rather than SPAT being preset (e.g., green light duration based on time of day) or based on actuation sensors that are only triggered once a vehicle has crossed or stopped at the intersection, the infrastructure could adjust the SPAT based on vehicle-communicated traffic volume. Not only would this allow signals to favor sections with the heaviest congestion, it would also permit more fluid adjustments for unusual or unexpected changes in traffic patterns. Increased congestion due to a special event, construction, or nearby accident could be alleviated more easily without the need for manual adjustment of the signal control network or police assistance.

Beyond the scope of this analysis, the DSRC capabilities of CACC technology could also enable safety benefits along an arterial corridor. I2V communications could warn drivers of intersecting vehicles appearing to be running a red light, pedestrian crosswalk activations, approaching emergency vehicles, etc.