Lane Change design

The Lane Change module is activated when lane change is needed and can be safely executed.

Lane Change Requirement

• As the prerequisite, the type of lane boundary in the HD map has to be one of the following:
  • Dashed lane marking: Lane changes are permitted in both directions.
  • Dashed marking on the left and solid on the right: Lane changes are allowed from left to right.
  • Dashed marking on the right and solid on the left: Lane changes are allowed from right to left.
  • allow_lane_change tags is set as true
• During lane change request condition
  • The ego-vehicle isn’t on a preferred_lane.
  • There is neither intersection nor crosswalk on the path of the lane change
• lane change ready condition
  • Path of the lane change does not collide with other dynamic objects (see the figure below)
  • Lane change candidate path is approved by an operator.

Generating Lane Change Candidate Path

The lane change candidate path is divided into two phases: preparation and lane-changing. The following figure illustrates each phase of the lane change candidate path.

Preparation phase

The preparation trajectory is the candidate path's first and the straight portion generated along the ego vehicle's current lane. The length of the preparation trajectory is computed as follows.

lane_change_prepare_distance = current_speed * lane_change_prepare_duration + 0.5 * deceleration * lane_change_prepare_duration^2

During the preparation phase, the turn signal will be activated when the remaining distance is equal to or less than lane_change_search_distance.

Lane-changing phase

The lane-changing phase consist of the shifted path that moves ego from current lane to the target lane. Total distance of lane-changing phase is as follows. Note that during the lane changing phase, the ego vehicle travels at a constant speed.

lane_change_prepare_velocity = std::max(current_speed + deceleration * lane_change_prepare_duration, minimum_lane_changing_velocity)
lane_changing_distance = lane_change_prepare_velocity * lane_changing_duration

The backward_length_buffer_for_end_of_lane is added to allow some window for any possible delay, such as control or mechanical delay during brake lag.

Multiple candidate path samples (longitudinal acceleration)

Lane change velocity is affected by the ego vehicle's current velocity. High velocity requires longer preparation and lane changing distance. However we also need to plan lane changing trajectories in case ego vehicle slows down. Computing candidate paths that assumes ego vehicle's slows down is performed by substituting predetermined deceleration value into prepare_length, prepare_velocity and lane_changing_length equation.

The predetermined longitudinal acceleration values are a set of value that starts from longitudinal_acceleration = maximum_longitudinal_acceleration, and decrease by longitudinal_acceleration_resolution until it reaches longitudinal_acceleration = -maximum_longitudinal_deceleration. Both maximum_longitudinal_acceleration and maximum_longitudinal_deceleration are calculated as: defined in the common.param file as normal.min_acc.

maximum_longitudinal_acceleration = min(common_param.max_acc, lane_change_param.max_acc)
maximum_longitudinal_deceleration = max(common_param.min_acc, lane_change_param.min_acc)

where common_param is vehicle common parameter, which defines vehicle common maximum longitudinal acceleration and deceleration. Whereas, lane_change_param has maximum longitudinal acceleration and deceleration for the lane change module. For example, if a user set and common_param.max_acc=1.0 and lane_change_param.max_acc=0.0, maximum_longitudinal_acceleration becomes 0.0, and the lane change does not accelerate in the lane change phase.

The longitudinal_acceleration_resolution is determine by the following

longitudinal_acceleration_resolution = (maximum_longitudinal_acceleration - minimum_longitudinal_acceleration) / longitudinal_acceleration_sampling_num

Note that when the current_velocity is lower than minimum_lane_changing_velocity, the vehicle needs to accelerate its velocity to minimum_lane_changing_velocity. Therefore, longitudinal acceleration becomes positive value (not decelerate).

The chart illustrates the conditions under which longitudinal acceleration values are sampled.

while the following describes the process by which longitudinal accelerations are sampled.

The following figure illustrates when longitudinal_acceleration_sampling_num = 4. Assuming that maximum_deceleration = 1.0 then a0 == 0.0 == no deceleration, a1 == 0.25, a2 == 0.5, a3 == 0.75 and a4 == 1.0 == maximum_deceleration. a0 is the expected lane change trajectories should ego vehicle do not decelerate, and a1's path is the expected lane change trajectories should ego vehicle decelerate at 0.25 m/s^2.

Which path will be chosen will depend on validity and collision check.

Multiple candidate path samples (lateral acceleration)

In addition to sampling longitudinal acceleration, we also sample lane change paths by adjusting the value of lateral acceleration. Since lateral acceleration influences the duration of a lane change, a lower lateral acceleration value results in a longer lane change path, while a higher lateral acceleration value leads to a shorter lane change path. This allows the lane change module to generate a shorter lane change path by increasing the lateral acceleration when there is limited space for the lane change.

The maximum and minimum lateral accelerations are defined in the lane change parameter file as a map. The range of lateral acceleration is determined for each velocity by linearly interpolating the values in the map. Let's assume we have the following map

Ego Velocity	Minimum lateral acceleration	Maximum lateral acceleration
0.0	0.2	0.3
2.0	0.2	0.4
4.0	0.3	0.4
6.0	0.3	0.5

In this case, when the current velocity of the ego vehicle is 3.0, the minimum and maximum lateral accelerations are 0.25 and 0.4 respectively. These values are obtained by linearly interpolating the second and third rows of the map, which provide the minimum and maximum lateral acceleration values.

Within this range, we sample the lateral acceleration for the ego vehicle. Similar to the method used for sampling longitudinal acceleration, the resolution of lateral acceleration (lateral_acceleration_resolution) is determined by the following:

lateral_acceleration_resolution = (maximum_lateral_acceleration - minimum_lateral_acceleration) / lateral_acceleration_sampling_num

Candidate Path's validity check

A candidate path is considered valid if it meets the following criteria:

1. The distance from the ego vehicle's current position to the end of the current lanes is sufficient to perform a single lane change.
2. The distance from the ego vehicle's current position to the goal along the current lanes is adequate to complete multiple lane changes.
3. The distance from the ego vehicle's current position to the end of the target lanes is adequate for completing multiple lane changes.
4. Intersection requirements are met (conditions are parameterized).
5. Crosswalk requirements are satisfied (conditions are parameterized).
6. Traffic light regulations are adhered to (conditions are parameterized).
7. The lane change can be completed after passing a parked vehicle.
8. The lane change is deemed safe to execute.

The following flow chart illustrates the validity check.

Candidate Path's Safety check

See safety check utils explanation

Objects selection and classification

First, we divide the target objects into obstacles in the target lane, obstacles in the current lane, and obstacles in other lanes. Target lane indicates the lane that the ego vehicle is going to reach after the lane change and current lane mean the current lane where the ego vehicle is following before the lane change. Other lanes are lanes that do not belong to the target and current lanes. The following picture describes objects on each lane. Note that users can remove objects either on current and other lanes from safety check by changing the flag, which are check_objects_on_current_lanes and check_objects_on_other_lanes.

Furthermore, to change lanes behind a vehicle waiting at a traffic light, we skip the safety check for the stopping vehicles near the traffic light. The explanation for parked car detection is written in documentation for avoidance module.

The detection area for the target lane can be expanded beyond its original boundaries to enable detection of objects that are outside the target lane's limits.

Without Lane Expansion

With Lane Expansion

Collision check in prepare phase

The ego vehicle may need to secure ample inter-vehicle distance ahead of the target vehicle before attempting a lane change. The flag enable_collision_check_at_prepare_phase can be enabled to gain this behavior. The following image illustrates the differences between the false and true cases.

The parameter prepare_phase_ignore_target_speed_thresh can be configured to ignore the prepare phase collision check for targets whose speeds are less than a specific threshold, such as stationary or very slow-moving objects.

If the lane is blocked and multiple lane changes

When driving on the public road with other vehicles, there exist scenarios where lane changes cannot be executed. Suppose the candidate path is evaluated as unsafe, for example, due to incoming vehicles in the adjacent lane. In that case, the ego vehicle can't change lanes, and it is impossible to reach the goal. Therefore, the ego vehicle must stop earlier at a certain distance and wait for the adjacent lane to be evaluated as safe. The minimum stopping distance can be computed from shift length and minimum lane changing velocity.

lane_changing_time = f(shift_length, lat_acceleration, lat_jerk)
minimum_lane_change_distance = minimum_prepare_length + minimum_lane_changing_velocity * lane_changing_time + lane_change_finish_judge_buffer

The following figure illustrates when the lane is blocked in multiple lane changes cases.

Stopping position when an object exists ahead

When an obstacle is in front of the ego vehicle, stop with keeping a distance for lane change. The position to be stopped depends on the situation, such as when the lane change is blocked by the target lane obstacle, or when the lane change is not needed immediately.The following shows the division in that case.

When the ego vehicle is near the end of the lane change

Regardless of the presence or absence of objects in the lane change target lane, stop by keeping the distance necessary for lane change to the object ahead.

When the ego vehicle is not near the end of the lane change

If there are NO objects in the lane change section of the target lane, stop by keeping the distance necessary for lane change to the object ahead.

If there are objects in the lane change section of the target lane, stop WITHOUT keeping the distance necessary for lane change to the object ahead.

When the target lane is far away

When the target lane for lane change is far away and not next to the current lane, do not keep the distance necessary for lane change to the object ahead.

Lane Change When Stuck

The ego vehicle is considered stuck if it is stopped and meets any of the following conditions:

• There is an obstacle in front of the current lane
• The ego vehicle is at the end of the current lane

In this case, the safety check for lane change is relaxed compared to normal times. Please refer to the 'stuck' section under the 'Collision checks during lane change' for more details. The function to stop by keeping a margin against forward obstacle in the previous section is being performed to achieve this feature.

Lane change regulations

If you want to regulate lane change on crosswalks or intersections, the lane change module finds a lane change path excluding it includes crosswalks or intersections. To regulate lane change on crosswalks or intersections, change regulation.crosswalk or regulation.intersection to true. If the ego vehicle gets stuck, to avoid stuck, it enables lane change in crosswalk/intersection. If the ego vehicle stops more than stuck_detection.stop_time seconds, it is regarded as a stuck. If the ego vehicle velocity is smaller than stuck_detection.velocity, it is regarded as stopping.

Aborting lane change

The abort process may result in three different outcome; Cancel, Abort and Stop/Cruise.

The following depicts the flow of the abort lane change check.

To preventive measure for lane change path oscillations caused by alternating safe and unsafe conditions, an additional hysteresis count check is implemented before executing an abort or cancel maneuver. If unsafe, the unsafe_hysteresis_count_ is incremented and compared against unsafe_hysteresis_threshold; exceeding it prompts an abort condition check, ensuring decisions are made with consideration to recent safety assessments as shown in flow chart above. This mechanism stabilizes decision-making, preventing abrupt changes due to transient unsafe conditions.

Cancel

Suppose the lane change trajectory is evaluated as unsafe. In that case, if the ego vehicle has not departed from the current lane yet, the trajectory will be reset, and the ego vehicle will resume the lane following the maneuver.

The function can be enabled by setting enable_on_prepare_phase to true.

The following image illustrates the cancel process.

Abort

Assume the ego vehicle has already departed from the current lane. In that case, it is dangerous to cancel the path, and it will cause the ego vehicle to change the heading direction abruptly. In this case, planning a trajectory that allows the ego vehicle to return to the current path while minimizing the heading changes is necessary. In this case, the lane change module will generate an abort path. The following images show an example of the abort path. Do note that the function DOESN'T GUARANTEE a safe abort process, as it didn't check the presence of the surrounding objects and/or their reactions. The function can be enable manually by setting both enable_on_prepare_phase and enable_on_lane_changing_phase to true. The parameter max_lateral_jerk need to be set to a high value in order for it to work.

Stop/Cruise

The last behavior will also occur if the ego vehicle has departed from the current lane. If the abort function is disabled or the abort is no longer possible, the ego vehicle will attempt to stop or transition to the obstacle cruise mode. Do note that the module DOESN'T GUARANTEE safe maneuver due to the unexpected behavior that might've occurred during these critical scenarios. The following images illustrate the situation.

Parameters

Essential lane change parameters

The following parameters are configurable in lane_change.param.yaml

Name	Unit	Type	Description	Default value
backward_lane_length	[m]	double	The backward length to check incoming objects in lane change target lane.	200.0
prepare_duration	[m]	double	The preparation time for the ego vehicle to be ready to perform lane change.	4.0
backward_length_buffer_for_end_of_lane	[m]	double	The end of lane buffer to ensure ego vehicle has enough distance to start lane change	3.0
backward_length_buffer_for_blocking_object	[m]	double	The end of lane buffer to ensure ego vehicle has enough distance to start lane change when there is an object in front	3.0
lane_change_finish_judge_buffer	[m]	double	The additional buffer used to confirm lane change process completion	2.0
finish_judge_lateral_threshold	[m]	double	Lateral distance threshold to confirm lane change process completion	0.2
lane_changing_lateral_jerk	[m/s3]	double	Lateral jerk value for lane change path generation	0.5
minimum_lane_changing_velocity	[m/s]	double	Minimum speed during lane changing process.	2.78
prediction_time_resolution	[s]	double	Time resolution for object's path interpolation and collision check.	0.5
longitudinal_acceleration_sampling_num	[-]	int	Number of possible lane-changing trajectories that are being influenced by longitudinal acceleration	3
lateral_acceleration_sampling_num	[-]	int	Number of possible lane-changing trajectories that are being influenced by lateral acceleration	3
object_check_min_road_shoulder_width	[m]	double	Width considered as a road shoulder if the lane does not have a road shoulder	0.5
object_shiftable_ratio_threshold	[-]	double	Vehicles around the center line within this distance ratio will be excluded from parking objects	0.6
min_length_for_turn_signal_activation	[m]	double	Turn signal will be activated if the ego vehicle approaches to this length from minimum lane change length	10.0
length_ratio_for_turn_signal_deactivation	[-]	double	Turn signal will be deactivated if the ego vehicle approaches to this length ratio for lane change finish point	0.8
max_longitudinal_acc	[-]	double	maximum longitudinal acceleration for lane change	1.0
min_longitudinal_acc	[-]	double	maximum longitudinal deceleration for lane change	-1.0
lateral_acceleration.velocity	[m/s]	double	Reference velocity for lateral acceleration calculation (look up table)	[0.0, 4.0, 10.0]
lateral_acceleration.min_values	[m/ss]	double	Min lateral acceleration values corresponding to velocity (look up table)	[0.4, 0.4, 0.4]
lateral_acceleration.max_values	[m/ss]	double	Max lateral acceleration values corresponding to velocity (look up table)	[0.65, 0.65, 0.65]

Lane change regulations

Name	Unit	Type	Description	Default value
regulation.crosswalk	[-]	boolean	Allow lane change in between crosswalks	true
regulation.intersection	[-]	boolean	Allow lane change in between intersections	true
regulation.traffic_light	[-]	boolean	Allow lane change to be performed in between traffic light	true

Ego vehicle stuck detection

Name	Unit	Type	Description	Default value
stuck_detection.velocity	[m/s]	double	Velocity threshold for ego vehicle stuck detection	0.1
stuck_detection.stop_time	[s]	double	Stop time threshold for ego vehicle stuck detection	3.0

Collision checks

Target Objects

Name	Unit	Type	Description	Default value
target_object.car	[-]	boolean	Include car objects for safety check	true
target_object.truck	[-]	boolean	Include truck objects for safety check	true
target_object.bus	[-]	boolean	Include bus objects for safety check	true
target_object.trailer	[-]	boolean	Include trailer objects for safety check	true
target_object.unknown	[-]	boolean	Include unknown objects for safety check	true
target_object.bicycle	[-]	boolean	Include bicycle objects for safety check	true
target_object.motorcycle	[-]	boolean	Include motorcycle objects for safety check	true
target_object.pedestrian	[-]	boolean	Include pedestrian objects for safety check	true

common

Name	Unit	Type	Description	Default value
safety_check.lane_expansion.left_offset	[m]	double	Expand the left boundary of the detection area, allowing objects previously outside on the left to be detected and registered as targets.	0.0
safety_check.lane_expansion.right_offset	[m]	double	Expand the right boundary of the detection area, allowing objects previously outside on the right to be detected and registered as targets.	0.0

Additional parameters

Name	Unit	Type	Description	Default value
enable_collision_check_for_prepare_phase.general_lanes	[-]	boolean	Perform collision check starting from the prepare phase for situations not explicitly covered by other settings (e.g., intersections). If false, collision check only evaluated for lane changing phase.	false
enable_collision_check_for_prepare_phase.intersection	[-]	boolean	Perform collision check starting from prepare phase when ego is in intersection. If false, collision check only evaluated for lane changing phase.	true
enable_collision_check_for_prepare_phase.turns	[-]	boolean	Perform collision check starting from prepare phase when ego is in lanelet with turn direction tags. If false, collision check only evaluated for lane changing phase.	true
prepare_phase_ignore_target_speed_thresh	[m/s]	double	Ignore collision check in prepare phase of object speed that is lesser that the configured value. enable_collision_check_at_prepare_phase must be true	0.1
check_objects_on_current_lanes	[-]	boolean	If true, the lane change module check objects on current lanes when performing collision assessment.	false
check_objects_on_other_lanes	[-]	boolean	If true, the lane change module include objects on other lanes. when performing collision assessment	false
use_all_predicted_path	[-]	boolean	If false, use only the predicted path that has the maximum confidence.	true

safety constraints during lane change path is computed

Name	Unit	Type	Description	Default value
safety_check.execution.expected_front_deceleration	[m/s^2]	double	The front object's maximum deceleration when the front vehicle perform sudden braking. (*1)	-1.0
safety_check.execution.expected_rear_deceleration	[m/s^2]	double	The rear object's maximum deceleration when the rear vehicle perform sudden braking. (*1)	-1.0
safety_check.execution.rear_vehicle_reaction_time	[s]	double	The reaction time of the rear vehicle driver which starts from the driver noticing the sudden braking of the front vehicle until the driver step on the brake.	2.0
safety_check.execution.rear_vehicle_safety_time_margin	[s]	double	The time buffer for the rear vehicle to come into complete stop when its driver perform sudden braking.	1.0
safety_check.execution.lateral_distance_max_threshold	[m]	double	The lateral distance threshold that is used to determine whether lateral distance between two object is enough and whether lane change is safe.	2.0
safety_check.execution.longitudinal_distance_min_threshold	[m]	double	The longitudinal distance threshold that is used to determine whether longitudinal distance between two object is enough and whether lane change is safe.	3.0
safety_check.cancel.longitudinal_velocity_delta_time	[m]	double	The time multiplier that is used to compute the actual gap between vehicle at each predicted points (not RSS distance)	0.8

safety constraints to cancel lane change path

Name	Unit	Type	Description	Default value
safety_check.cancel.expected_front_deceleration	[m/s^2]	double	The front object's maximum deceleration when the front vehicle perform sudden braking. (*1)	-1.0
safety_check.cancel.expected_rear_deceleration	[m/s^2]	double	The rear object's maximum deceleration when the rear vehicle perform sudden braking. (*1)	-2.0
safety_check.cancel.rear_vehicle_reaction_time	[s]	double	The reaction time of the rear vehicle driver which starts from the driver noticing the sudden braking of the front vehicle until the driver step on the brake.	1.5
safety_check.cancel.rear_vehicle_safety_time_margin	[s]	double	The time buffer for the rear vehicle to come into complete stop when its driver perform sudden braking.	0.8
safety_check.cancel.lateral_distance_max_threshold	[m]	double	The lateral distance threshold that is used to determine whether lateral distance between two object is enough and whether lane change is safe.	1.0
safety_check.cancel.longitudinal_distance_min_threshold	[m]	double	The longitudinal distance threshold that is used to determine whether longitudinal distance between two object is enough and whether lane change is safe.	2.5
safety_check.cancel.longitudinal_velocity_delta_time	[m]	double	The time multiplier that is used to compute the actual gap between vehicle at each predicted points (not RSS distance)	0.6

safety constraints used during lane change path is computed when ego is stuck

Name	Unit	Type	Description	Default value
safety_check.stuck.expected_front_deceleration	[m/s^2]	double	The front object's maximum deceleration when the front vehicle perform sudden braking. (*1)	-1.0
safety_check.stuck.expected_rear_deceleration	[m/s^2]	double	The rear object's maximum deceleration when the rear vehicle perform sudden braking. (*1)	-1.0
safety_check.stuck.rear_vehicle_reaction_time	[s]	double	The reaction time of the rear vehicle driver which starts from the driver noticing the sudden braking of the front vehicle until the driver step on the brake.	2.0
safety_check.stuck.rear_vehicle_safety_time_margin	[s]	double	The time buffer for the rear vehicle to come into complete stop when its driver perform sudden braking.	1.0
safety_check.stuck.lateral_distance_max_threshold	[m]	double	The lateral distance threshold that is used to determine whether lateral distance between two object is enough and whether lane change is safe.	2.0
safety_check.stuck.longitudinal_distance_min_threshold	[m]	double	The longitudinal distance threshold that is used to determine whether longitudinal distance between two object is enough and whether lane change is safe.	3.0
safety_check.stuck.longitudinal_velocity_delta_time	[m]	double	The time multiplier that is used to compute the actual gap between vehicle at each predicted points (not RSS distance)	0.8

(*1) the value must be negative.

Abort lane change

The following parameters are configurable in lane_change.param.yaml.

Name	Unit	Type	Description	Default value
cancel.enable_on_prepare_phase	[-]	boolean	Enable cancel lane change	true
cancel.enable_on_lane_changing_phase	[-]	boolean	Enable abort lane change.	false
cancel.delta_time	[s]	double	The time taken to start steering to return to the center line.	3.0
cancel.duration	[s]	double	The time taken to complete returning to the center line.	3.0
cancel.max_lateral_jerk	[m/sss]	double	The maximum lateral jerk for abort path	1000.0
cancel.overhang_tolerance	[m]	double	Lane change cancel is prohibited if the vehicle head exceeds the lane boundary more than this tolerance distance	0.0
unsafe_hysteresis_threshold	[-]	int	threshold that helps prevent frequent switching between safe and unsafe decisions	10

Debug

The following parameters are configurable in lane_change.param.yaml.

Name	Unit	Type	Description	Default value
publish_debug_marker	[-]	boolean	Flag to publish debug marker	false

Debug Marker & Visualization

To enable the debug marker, execute (no restart is needed)

ros2 param set /planning/scenario_planning/lane_driving/behavior_planning/behavior_path_planner lane_change.publish_debug_marker true

or simply set the publish_debug_marker to true in the lane_change.param.yaml for permanent effect (restart is needed).

Then add the marker

/planning/scenario_planning/lane_driving/behavior_planning/behavior_path_planner/debug/lane_change_left

in rviz2.

Available information

1. Ego to object relation, plus safety check information
2. Ego vehicle interpolated pose up to the latest safety check position.
3. Object is safe or not, shown by the color of the polygon (Green = Safe, Red = unsafe)
4. Valid candidate paths.
5. Position when lane changing start and end.