A jewish law web site says (see link):

The position of Jewish law in such a case is well known, being based on the biblical imperative "lo ta'amod al dam re'echa" (Leviticus 19:16) - a person must not stand idly by while he sees his fellow Jew in danger of life or limb.

This is pretty much what I was thinking. The KJV says "stand aside", while the text from the web site has "stand idly by".

My interest in it came from looking at the good samaritan. If this idea is (and was) "well known", then the priest and levite knew they were obligated to stop and help.

I don't know about that particular meaning in this verse (Lev. 19:16), but the verse is in a section that deals with overall good demeanor and responsible behavior toward your neighbor, including "love your neighbor as yourself" in Lev. 19:18. Your thought immediately brought to mind "Love does no harm to its neighbor. Therefore love is the fulfillment of the law." (Romans 13:10 NIV) and "Anyone, then, who knows the good he ought to do and doesn't do it, sins." (James 4:17 NIV) along with "Dear children, let us not love with words or tongue but with actions and in truth." (1 John 3:18 NIV) These principles are embodied in instructions like Ex. 23:4 and Deut. 5:14 and surely stand behind the injunction in Lev. 19:17.