planning: change following distance only considers ADC speed.

modules/planning/common/planning_gflags.cc

@@ -113,6 +113,9 @@ DEFINE_double(nudge_distance_obstacle, 0.3,
               "minimum distance to nudge a obstacle (meters)");
 DEFINE_double(follow_min_distance, 10,
               "min follow distance for vehicles/bicycles/moving objects");
+DEFINE_double(
+    follow_time_buffer, 4.0,
+    "follow time buffer (in second) to calculate the following distance.");
 DEFINE_double(stop_line_min_distance, 0.0,
               "min distance (meters) to stop line for a valid stop");
 

modules/planning/common/planning_gflags.h

@@ -75,6 +75,8 @@ DECLARE_double(stop_distance_destination);
 DECLARE_double(min_driving_width);
 DECLARE_double(nudge_distance_obstacle);
 DECLARE_double(follow_min_distance);
+DECLARE_double(follow_time_buffer);
+
 DECLARE_double(stop_line_min_distance);
 
 DECLARE_string(destination_obstacle_id);

modules/planning/tasks/dp_st_speed/dp_st_graph.cc

@@ -496,13 +496,9 @@ bool DpStGraph::CreateFollowDecision(
 
   auto* follow = follow_decision->mutable_follow();
 
-  // in seconds
-  constexpr double kFollowTimeBuffer = 4.0;
-  const auto& velocity = path_obstacle.obstacle()->Perception().velocity();
-  const double follow_speed =
-      std::fmax(init_point_.v(), std::hypot(velocity.x(), velocity.y()));
+  const double follow_speed = init_point_.v();
   const double follow_distance_s =
-      -std::fmax(follow_speed * kFollowTimeBuffer, FLAGS_follow_min_distance);
+      -std::fmax(follow_speed * FLAGS_follow_time_buffer, FLAGS_follow_min_distance);
 
   follow->set_distance_s(follow_distance_s);