Much symbolism insults the intelligence. Objectivity is missing in many interpretations and the inconsistency of meaning across different cultures refutes the alleged 'universality' of many symbols.
As a means of communication, symbols are valid if they bear a recognisable resemblance to that which they symbolise. The stylised male and female figures for washroom signage are examples of coherent symbology; the red rose symbolising romantic love is an example of the opposite. The association between the rose and romance is weighted with cultural and historical chains. Such a 'symbol' would be incoherent to an outsider who isn't initiated in the culture-specific visual language that is the context for the 'rose for romance' symbolism.
Coherent symbols are truly universal because they derive their form and meaning from objective reality, while limiting subjective interpretation based on a whim. Such symbols are the common property of all humanity for their validity does not depend on the cultural or charismatic power of any particular society, individual or organisation. No single creator or group of creators can claim authorship of a universal symbol, unlike the corporate logos that saturate the physical and mental environment. To put it another way, a society, individual or organisation may have the power to propagate incoherent symbols whose meanings are dictated by their creators (or owners, as with corporate logos that 'symbolise' the company's carefully constructed image and associated values), but coherent symbols and their meanings are independent of anyone's agenda. No person, group, government or corporation is powerful enough to switch around the meanings of the male and female washroom signs (at least without intending ironic humour), or convince people that the sign depicting a stylised airplane on the highway represents a train-station.