Highway advisory radio

Travelers information stations (TIS), also called highway advisory radio stations (HAR) by the United States Department of Transportation, are licensed low-power AM radio stations operated by departments of transportation, airports, local government, colleges, parks, events and destinations. The stations provide information to motorists regarding travel, situations of imminent danger and emergencies.

These systems are licensed by the Federal Communications Commission (FCC) in the United States. The TIS station class was originally created by the FCC by the Report and Order in Docket 20509.

TIS stations are limited to a maximum signal of 2 mV/m at 1.5 km (0.93 mi) using a power of up to 10 watts to achieve this limit in the case of vertical antenna systems (the most common type). Up to 50 watts in the case of the radiating cable antenna systems may be used to achieve a maximum of 2 mV/m at 60 m (200 ft) from the radiating cable. Radiating cable systems are limited to a continuous antenna length of 1.9 km (1.2 mi). Cable systems are used for situations such as Dulles International Airport with very long limited-access approach roads where multiple systems can be placed in 1.9 km segments.

Critical evacuation systems, such as those in the Florida Keys and near chemical and nuclear facilities, have been granted exceptional power waivers for emergency operations. These systems will typically operate under the normal power level, but have permission to exceed that limit, typically to 100 W, in the face of a critical emergency evacuation situation. The former American Airlines TIS stations at Dallas-Fort Worth International Airport on 1640 and 1680 kHz (before the AM band expanded above 1600 kHz) ran 60 watts power out under special exemption.

The station audio is required to be passed through an audio low-pass filter which rolls off frequencies above 3 kHz, to prevent music from being played as a low power AM station. There have been discussions by the FCC about whether to remove this requirement as it has been found that it is not effective in reducing interference to non-TIS stations (the original reasoning for the rule) and it degrades the intelligibility of the station audio.

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