During the last decades the western society is constantly confronted with problems caused by increasing road traffic. This increase in traffic demand leads to congestion and has a negative effect on traffic safety, environment and energy consumption. The expectations of the use of telematics technology in road traffic in this respect are high, since this technology could lead to system innovations, which in the long term can contribute to the problems faced. In the coming years motorists will have at their command a range of Intelligent Transport Systems (ITS). ITS that support the driver in performing the driving tasks are called Advanced Driver Assistance (ADA) systems. An ADA system that has been introduced on a small scale by the automotive industry is Autonomous Adaptive Cruise Control (AACC). AACC is a radar-based system which is designed to enhance driving comfort and convenience on highways by relieving the driver of the need to continuously adjust his or her speed to match that of the preceding vehicle while also maintaining a proper headway. Vehicle-to-vehicle communication is potentially boosting the development of ADA systems. Recently, vehicle-to-vehicle communication is added to the AACC system. This system is called Co-operative Adaptive Cruise Control, abbreviated CACC.

Cruise control systemsVehicle-to-vehicle communication could provide an ACC system with more and better information about the vehicle it is following, enabling the own vehicle to react faster and smoother on acceleration and deceleration of the predecessor. Since communication is quicker, more reliable and less noisy than autonomous sensing, significant closer headways (down to a time gap that could be as low as 0.5 second) can be postulated. Although CACC is primarily designed for giving the driver more comfort and convenience, the system potentially has effects on traffic flow and highway capacity when it is widely used. However, since CACC is in a very early stage of development, not much research into the traffic flow impacts of CACC has been done so far. This study aims at assessing the traffic flow impacts of CACC on a Dutch highway. Since CACC does not exist yet and therefore field tests on highways are not an option, research has been done with the microscopic traffic simulation model ‘MIXIC’. MIXIC describes the behaviour of vehicles on highways in a very detailed way. The functionality of CACC has been elaborated in functional specifications for MIXIC and the model is extended with this functionality.

Experiments with a platoon of CACC equipped vehicles approaching a slower vehicle on a single lane indicate that if these vehicles following each other in the CACC mode the time gap between these vehicles can safely be decreased to as small as 0.5 second. Further, the experiments demonstrate an improvement of the string stability compared to a platoon of manually driven vehicles. String stability is stability with respect to inter-vehicular spacing. A very small, almost negligible communication delay replaces the reaction delay of the driver and the sensor delay of the autonomous ACC. This makes the CACC controller smoother and more natural in response.

MIXIC simulations performed with data measured on a 4-lane Dutch highway with a bottleneck due to a road narrowing prove that CACC has the ability to improve traffic flow performance. However, to what extent depends heavily on the traffic flow conditions on the motorway stretch and the CACC penetration rate. In high flow conditions there is more interaction between vehicles. As a result more vehicles are able to participate in a CACC platoon (if they are CACC equipped). Since CACC reduces time headways and improves string stability, traffic flow improves stronger in high traffic flow conditions and a high CACC penetration rate.Traffic flow stability improves with an increasing CACC penetration rate, which is measured as a reduction of the number of shock waves, varying from 25% (20% CACC) to 90% (100% CACC) on the high-intensity stretch before a bottleneck. If the CACC penetration rate is low (<40%), this improvement in traffic stability does not result in a better throughput. Even slightly lower average speeds are measured on all links. If the largest part of the vehicle fleet is CACC equipped (>60%) an improved traffic flow performance with respect to the reference case of only manually driven vehicles is established (up to 10% improvement on the stretch before the bottleneck).

The impact of a dedicated lane for CACC equipped vehicles (CACC-lane) depends heavily on the CACC penetration rate. A low CACC presence (<40%) leads to a degradation of performance, which is demonstrated by lower speeds, higher speed variances and more shock waves. This is explained by an increase of the intensities on the regular lanes and a relatively high number of lane-changes, because manually driven vehicles should leave the left lane before it turns into a CACC-lane. As a result an unbalanced division of vehicles over the lanes is observed. The potential improvement of traffic performance of a CACC-lane with respect to a highway configuration without CACC-lane is only confirmed for the high-intensity stretch before the bottleneck if CACC fractions are high (>50%).

As communication is restricted to longitudinal control and no restrictions to the length and compactness of CACC platoons is given, the system has negative effects in the merging process. Close CACC platoons prevent other vehicles from cutting in, because the gap between consecutive CACC vehicles is smaller than the gap accepted for performing a mandatory lane change. The unsafe lateral effect of the presence of CACC in traffic is revealed by an increasing number of removed vehicles from the simulation due to conflicts as more vehicles are CACC equipped. It is recommended to study possible solutions for dealing with this negative effect on merging. Options for improvement are (i) limiting the length of a CACC platoon, (ii) placing an infrastructural beacon on the motorway-stretch before a bottleneck that communicates to a vehicle that it should enlarge the time gap to its predecessor so a vehicle which wants to cut in can perform its lane change and (iii) adding a ‘Co-operative Merging’ application to the CACC system enabling vehicles to communicate their lateral movements (e.g. lane-change).

Implementing the CACC system is not a simple matter of demonstrating technological progress and traffic improvement, but requires user-acceptance considering legal and liability issues, comfort and socio-economic impacts. Therefore, it is of importance to evaluate introduction effects already at an early stage of system development. An explorative study into the context of the CACC system and a stakeholder analysis has been performed in order to recognise driving forces and barriers that may affect a prospective CACC deployment. Stakeholders who are involved in the development of ADA systems in general and CACC in specific have different interests in this system and as a result take their own position towards CACC development and deployment. Different conditions for a successful market introduction of CACC are identified:


Considering human factors, driver awareness and driver’s wishes in the development of CACC.


Clarifying impacts of CACC by intensifying research.


Getting autonomous ACC to a stage where the system is widely recognised and used.


Developing of strategies to deal with the low usability in the early stage of deployment.


Agreeing on standards for a reliable communication system.