The major use case for the lane change assistant is the changing between lanes on multi-lane highways (cf. FR_1_1 in Table 3.1). For this use case, the assistant has to determine the current driving domain (FR_1). The lane change assistant is only developed for highways as part of a highway pilot and is currently unable to safely cope with the complexity of traffic in other domains, e.g., rural and urban roads.
On highways, the lane change assistant has to independently change from the velocity and available gap sizes between vehicles. The assistant has to perform lane changes in traffic jams with small gaps between vehicles and at slow velocities (FR_1_2) as well as in normal traffic with high differences in their velocities but large gaps between vehicles (FR_1_1). In normal traffic, the automated vehicle must drive faster than 10 m/s (FR_2_1). Otherwise, the overtaking would take too much time and compromise other traffic participants. Additionally, a lane change is not supposed to be executed in curves with a curvature less than 125 m because actuators might not be able to execute lane changes safely (cf. [Vis+08]). The automated vehicle would leave the target lane (FR_2_2). Before every lane change, the automated vehicle has to be properly aligned inside its current lane in order to guarantee the correct and safe execution of the lane changes. A lane change is not supposed to be executed if the position of the automated ego vehicle deviates more than 0.4 m from the center of its current lane (FR_2_3). A special case for lane changes on multi-lane highways is the merging with other traffic participants in front of constrictions. In general, the traffic in front of constrictions is slow and dense because the traffic of two or more lanes has to be condensed to fewer lanes. A lane change is inevitable for the automated vehicle if it drives on a lane with the constriction. As stated in requirement FR_4 (cf. Table 3.1), the automated vehicle must be able to perform alternate merging starting less than 100 m in front of the constriction. The constrictions limit the time and space available for the lane change.
3.2. Requirements Analysis
Table 3.1.: Requirements for the lane changing on highway. ID Description
FR_1 The system shall be able to detect if the current domain is a highway.
FR_1_1 The system shall be able to perform lane changes on multi-lane highways.
FR_1_2 The system shall be able to change lanes in a traffic jam with a traffic flow velocity of less than 5 m/s and gaps of less than 10 m between vehicles.
FR_2 The system has to prevent unsafe lane changes.
FR_2_1 The system shall be able to prevent lane changes if the ego velocity is less than 10 m/s.
FR_2_2 The system shall be able to prevent lane changes if the curva- ture of the ego lane is less than 125 m.
FR_2_3 The system shall be able to prevent lane changes if the lateral offset to the center of the current lane is more than 0.4 m. FR_3 The system shall be able to adjust the vehicle longitudinally
towards a suitable gap in a search range of 100 m to the front and 100 m to the rear of the vehicle in order to prepare a lane change.
FR_4 The system shall be able to perform lane changes with less than 100 m in front of constrictions following the alternate merging.
Lane changes in dense traffic require a high degree of cooperation between traffic partici- pants because there might be small to no gaps between vehicles for a lane change. For safe lane changes in dense traffic, the lane change assistant has to find an appropriate gap between vehicles on the target lane and position itself next to this gap in order to demand the cooperation of other traffic participants (FR_3). This requirement does not only apply to the merging in front of constriction but also to lane changes between highway lanes in dense traffic and to entering and leaving of highways — especially in weaving areas of interchanges (cf. Section 3.2.1.2).
Lane changes on highways are supposed to improve or sustain the progress of the automated ego vehicle towards its target destination. Therefore, each lane change has to be evaluated under consideration of its benefit for the overall progress of the vehicle’s journey (cf. FR_5_1 in Table 3.2). The evaluation of each lane changes has to consider the cost and distance ratio towards the target destination (FR_5_1_1) under consideration of dynamic traffic (FR_5_1_2) and timing restriction from prior events (FR_5_1_3). In front of constrictions, a benefit estimation is not necessary because a
Table 3.2.: Requirements for the benefit of lane changes.
ID Description
FR_5 The system must decide if a lane change is beneficial.
FR_5_1 In case, the system is in the automated driving mode, the system has to determine if a lane change is beneficial based on the current traffic situation.
FR_5_1_1 The system shall be able to determine whether a lane change is beneficial based on a cost / distance metric to follow the route towards the navigation destination.
FR_5_1_2 The system shall be able to assess the dynamic traffic situation whether changing the lanes would result in a dynamic benefit. FR_5_1_3 The system shall be able to assess, whether a lane change is beneficial based on timing restrictions regarding prior driving events.
Lane changes may have a short-term benefit but may result in a disadvantage in the mid or long-term run. For example, overtaking slower vehicles may lead to a short time advantage, but the traffic situation could later prohibit the automated vehicle to take its intended exit ramp. The short-term benefit for the overtaking has been negated by its mid- or long-term disadvantages resulting in an overall disadvantage.
Besides lane changes between highway lanes, the lane change assistant has to cope with lane changes to off-ramps and from on-ramps in order to enter and exit highways. The corresponding requirements for these use cases are presented in the following sections.