It looks like there is a conflict between the 2009 MUTCD and the Traffic Sign Handbook when discussing speed reduction and the correct use of the W2-5 (reduced speed limit) sign. Can you clarify?

MUTCD and Traffic Sign Handbook Covers

The Traffic Sign Handbook uses a different wording but remains consistent. The MUTCD states “reduced by more than 10 mph” while the sign handbook states “a reduction of 15 mph or more”

 

MUTCD
Section 2C.38 Reduced Speed Limit Ahead Signs (W3-5, W3-5a)
Guidance: “01 A Reduced Speed Limit Ahead (W3-5 or W3-5a) sign (see Figure 2C-7) should be used to inform road users of a reduced speed zone where the speed limit is being reduced by more than 10 mph, or where engineering judgment indicates the need for advance notice to comply with the posted speed limit ahead.”

Figure 2C-7 Reduced Speed Limit Ahead Signs

Reduced Speed Limit Signs

Traffic Sign Handbook
Pg. 90 Chapter 9 Speed Limits
“These signs are used to warn drivers that they are approaching a lower speed limit. They should be used anytime the speed limit reduces by 15 mph or more. They should also be used when the first sign for the lower speed limit cannot be seen in time to slow to the lower speed. The speed limit shown shall match the speed limit on the subsequent Speed Limit sign.”

To illustrate we created a quick table available here.

 

Resources

Traffic Sign Handbook
https://cornell.app.box.com/file/148603446136

MUTCD
https://mutcd.fhwa.dot.gov/htm/2009r1r2/part2/part2c.htm#section2C38

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