TTC related reward in function: "_on_highway_reward"

[lilin96]

if ttc != c_util.INFINITY and ttc < self.TTC_threshold:
if action == "accelerate":
r_lc = 1.2
reward += 1.2
elif action == "change_left" and tr_util.change_lane_chance(vehicleID=agent, change_direction=1):
r_lc = 0.6
reward += 0.6

     if action == "change_right" and tr_util.change_lane_chance(vehicleID=agent, change_direction=-1):
        r_lc = 0.3
         reward += 0.3
     else:
        r_lc = -1
        cost += 1

what is the meaning of these code? why there exists a front vehicle in current lane, if the controlled vehicle accelerate and then the reward increase?