HCM Analysis of the Potential Impacts of Driverless Vehicles on the Quality of Flow of Freeways and Intersections

Abstract

Autonomous or self-driving vehicles, popularly called “driverless cars” (DLC), are capable of navigating themselves through freeways and intersections without any human intervention. It is believed that DLC can bring about tremendous changes to our daily life, including safety, traffic efficiency, environment and community. By 2020 various companies will release their DLC models designed with different levels of automation. Traffic will be mixed with vehicles driven by human drivers and DLC, which cause uncertainties in the magnitude of impacts of DLC and their traffic shares. The Highway Capacity Manual (HCM) was modified to analyze the future impact of DLC on traffic flow operations. A review of the HCM parameters that might be directly affected by DLC is presented in this study. These parameters were modified to include the features of DLC: the technical capability which is quantified by the car-following headway of DLC, and the proportion of DLC in the traffic stream. Case studies were conducted to estimate the effects of DLC on the quality of traffic flow on a basic freeway segment, freeway weaving segment, signalized intersection, all-way stop-controlled intersection and roundabouts. For each case study, a sensitivity analysis was provided to evaluate the potential impacts of DLC with different technological capabilities under various traffic demands and proportions. The results from all case studies reflect the commonality of DLC impacts. DLC are able to maintain constant speed at a shorter car-following distance, which can smooth the traffic flow. For DLC that are connected with infrastructure and other DLC, the car-following distance will become shorter. With more connectivity built between DLC and infrastructure, the road space and delay will be saved by platooning. If in the future all vehicles are driverless cars, capacity of the road will be doubled or tripled. This means every road lane is capable of serving twice as many vehicles at no cost to the city. But if in the future there is only 1% of DLC in traffic, traffic flow will not improve. If the portion of DLC reaches 5%, congestion will improve by 5% if they are regular DLC, or by 12% if they are connected DLC. In the latter scenario, traffic flow can begin to improve in a noticeable way. However, if DLC are designed to drive conservatively, they will cause more delays than humans do.