text: #!/usr/bin/env python
  
  import rospy
  from geometry_msgs.msg import Twist
  
  cmd_vel_pub = rospy.Publisher('cmd_vel',Twist,queue_size = 1)
  rospy.init_node('red_light_green_light')
  
  red_light_twist = Twist()
  green_light_twist = Twist()
  green_light_twist.linear.x = 0.1
  
  driving_forward = False
  light_change_time = rospy.Time.now()
  rate = rospy.Rate(10)
  
  while not rospy.is_shutdown():
      if driving_forward:
          cmd_vel_pub.publish(green_light_twist)
      else:
          cmd_vel_pub.publish(red_light_twist)
  
      if light_change_time < rospy.Time.now():
          driving_forward = not driving_forward
          light_change_time = rospy.Time.now()+rospy.Duration(1)
      rate.sleep()
  