U.S. Department of Transportation Federal Highway Administration

February 22, 2005

In Reply Refer To: HSA-10

Mr. Kurt Smith, P.E.
State Traffic and Safety Engineer
Design and Construction Standards Section
3132 Channel Drive
Juneau, Alaska 99801-7898

Dear Mr. Smith:

Your February 4 letter was a follow-up to the previous day’s teleconference in which interpretation of a statement in the Federal Highway Administration’s current Technical Advisory (TA) T 5040.35, Roadway Shoulder Rumble Strips was discussed. Specifically, Section 9.b. (2) of this TA states, “The most recent studies indicate a milled depth of approximately 10 mm (3/8”) provides reasonable warning to most motorists while not being unduly dangerous to cross on a bicycle when necessary.” You expressed concern that this statement could be interpreted as suggesting 10-mm (3/8”) deep milled rumble strips were equivalent to 13-mm (1/2”) in effectiveness and that the deeper strips could be construed as being “unduly dangerous” to cyclists.

Numerous studies have shown conclusively that deeper rumble strips produce more noise and vibration when a vehicle crosses them. The logical conclusion is that the more aggressive a rumble strip design is, the more effective it will be at reducing the drift-off roadway crash that rumble strips are intended to address. These same studies have also shown the aggressive rumble strip designs can be a serious nuisance at best, and even a possible danger, to cyclists crossing them.

The intent of Section 9.b. (2) of the TA was to suggest several compromise designs that could be used by an agency to balance both motorist and bicyclist needs. A compromise design, by definition, would suggest that neither party is fully satisfied. In the case of shallower (3/8-in deep or less vs. ½-inch deep), continuous rumble strips, motorists are likely to get a less effective warning when their vehicles drift onto it. However, the degree to which this warning may be reduced has not been established. The bicyclist compromise is that crossing from driving lane to shoulder will always be made across a rumble strip, albeit a less jarring one.

This section of the TA also suggests another potential compromise design, in which the rumble depth is maintained at ½”, but gaps are provided in the rumble pattern to allow for bicycle crossover movement. This compromise may be more appealing to agencies that want to install a vibrant warning strip, because rumble depth is maintained and the intermittent strips will still engage the typical distracted or drift-off vehicle that, also by definition, begins road departure at a shallow angle.

Sincerely yours,

/Original Signed by/

Richard D. Powers
Highway Engineer, Office of Safety Design
Office of Safety

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