Can we use signs with LEDs in the border to enhance conspicuity?
Yes, use of LEDs is one of the methods highlighted in MUTCD section 2A.15. Enhanced Conspicuity for Standard Signs
https://mutcd.fhwa.dot.gov/htm/2009r1r2/part2/part2a.htm#section2A15 – Item J
Please note that adding a solid header panel with a legend of NOTICE, STATE LAW is not allowed in New York State.
LEDs are allowed as per MUTCD section 2A.07 Retroreflectivity and Illumination.https://mutcd.fhwa.dot.gov/htm/2009r1r2/part2/part2a.htm#section2A07
Here is a link to an FHWA webpage highlighting the benefits and use of LEDs in signs.
Do not use them everywhere, but in places where the extra costs are worth the extra visibility.