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In the case of a runway with a magnetic heading of 10° in America the runway number ("1") isn't hugely distinguishable from the runway centreline IMHO, (for example see Washington Airport and San Francisco Airport).

Is there a reason why the USA doesn't add the leading zero like the EU (example Brussels Airport and RAF Waddington)?

Does the ICAO make any recommendations about this?

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Indeed USA deviates from ICAO:

5.2.2.4 Zeros are not used to precede single-digit runway markings. An optional configuration of the numeral 1 is available to designate a runway 1 and to prevent confusion with the runway centerline.

Source: USA AIP - GEN 1.7 Differences From ICAO SARPs

5.2.2.4 in ICAO SARPs Annex 14 (Aerodromes) is as follows (emphasis mine):

5.2.2.4 A runway designation marking shall consist of a two-digit number and on parallel runways shall be supplemented with a letter. On a single runway, dual parallel runways and triple parallel runways the two-digit number shall be the whole number nearest the one-tenth of the magnetic North when viewed from the direction of approach. On four or more parallel runways, one set of adjacent runways shall be numbered to the nearest one-tenth magnetic azimuth and the other set of adjacent runways numbered to the next nearest one-tenth of the magnetic azimuth. When the above rule would give a single digit number, it shall be preceded by a zero.

Shall in ICAO legalese denotes a standard.

The aforementioned optional configuration for "1" covers your concern regarding the centerline:

enter image description here

The numeral "1", when used alone, contains a horizontal stroke, as shown, to differentiate it from the runway centerline marking.

Source: FAA AC 150/5340-1

The rationale is not in the AIP; it's not always given. Deviations from SARPs are also very common, that's why there's an AIP section dedicated to them.

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  • $\begingroup$ But the question is, why did the US choose to change the standard? $\endgroup$
    – Ray Butterworth
    Nov 22, 2019 at 17:14
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    $\begingroup$ @RayButterworth: Who said USA changed it? More like kept doing what they were doing before the SARPs were written down. Check the linked list of differences, and add a dollar value to each. Fully adhering is super expensive -- especially for a country with a big aviation sector like USA -- and not always really beneficial (if it ain't broke...). $\endgroup$
    – user14897
    Nov 22, 2019 at 17:46
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    $\begingroup$ @RayButterworth it’s actually a two part question, why (if known) and what does ICAO say. The first half may be unanswerable, but this does a very good job of answering the second half in my opinion. $\endgroup$
    – Notts90
    Nov 22, 2019 at 17:47

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