What is a transponder?

A transponder is a small electronic device that attaches to the inside of the vehicle’s windshield.

A transponder allows trucks to bypass weigh stations by electronically verifying a truck’s legal weight, safety rating, and credentials.

Here is how the transponder works :

  • Each transponder has a unique serial number that is assigned to a specific vehicle in a roadside database.
  • A green light indicates the checks were all satisfactory and the vehicle may bypass the weigh station.
  • A red light indicates the driver needs to report to the weigh station for closer inspection.

After receiving a red or green light on the transponder, the light will continue to flash intermittently for approximately 15 minutes. There is nothing wrong with the transponder. This is built in protection for the carrier in case the driver is pulled over by law enforcement. The transponder signal allows the officer to verify the signal that was transmitted by the database.

Applying for a transponder

The Washington State Department of Transportation (WSDOT) sells transponders that can be used in NORPASS member and partner states with no monthly or annual cost.
To obtain an application, call 1-888-877-8567 or download an application
You will need to submit:
  • A completed application. One application may be used for multiple transponder purchases.
  • A copy of the vehicle cab card or registration. If purchasing multiple transponders a copy of the cab card or registration is required for each vehicle.
  • Send a check or money order made payable to the address on the application for the total cost of the transponder(s) purchase.

Once the application has been accepted, you will receive the transponder(s) in the mail.

A black/white label will be attached to each transponder. This label identifies the license plate number of the vehicle the transponder is assigned to.

Transponder Installation and Operation

 The transponder is designed to be mounted inside the cab, on the windshield where the driver can observe the transponder without removing his/her eyes from the road. The device should be mounted no more than 6 inches from the top or bottom of the windshield, and 2 inches to the right of center. (Please refer to Title 49 CFR Subpart D Section 393.60 (e) (1))
 Installation procedures
  1. Thoroughly clean the area of the windshield where the transponder adhesive strips are to be mounted.
  2. Peel the backing tape off of the mounting pads and simply attach the transponder to the windshield as shown above.  Press firmly and do not remove the transponder from the glass, as this weakens the adhesive.

Kapsch Transponders

 The red and green LED lights are located along the left side of the transponder when mounted on the windshield. When a signal is received, the red or green LED flashes and an audible signal will be heard:

    • Continuously for 5 seconds
    • Flashes once every second for 25 seconds

Flashes once every 10 seconds for 14.5 minutes


BY-PASS INDICATION: When given the proper authorization, the green light indicator and tone will be recognized and the driver is to bypass the station.

PULL-IN INDICATION: When authorization is not received the red light indicator and tone will be recognized and the driver is to pull into the station. – If a pull-in indication is ignored, there will be no indicator on the transponder. However the weigh station computer will signal enforcement of the illegal bypass.

NO INDICATION: If there is no indication given by the transponder, the driver must pull into an open weigh station or (if applicable) follow roadside directional signs.