Is fluorescent clothing effective?
There are several terms around this subject. A key thing here is reflectivity (or more specifically retroreflectivity), rather than fluorescence. Fluoresence is the property of a material where it absorbs light at one wavelength, e.g. ultraviolet sunlight, but then reflects back light from the visual spectrum. This is most useful in daylight conditions, using the sunlight to create more human visible light. 

Everything (well, almost everything) is reflective to some degree, mostly diffuse reflection where light hits a surface and scatters, but this reflection goes in many directions and breaks apart into many wavelengths. Alternatively there is  a property called retroreflection which reflects light back to its source, in contrast to mirror (or specular) reflection which just bounces off, as light does with a mirror, in a single opposing direction. Hence a person near the light source, e.g. a driver in a car, sees the retroreflective surface.  

Thus in terms of visibility, you don't just need flourescent material at night because there isn't enough light in the usable spectrum, so a yellow jacket isn't going to be any more useful than a white t-shirt. So while they'll both be more visible than something black, in order to stand out in a driver's headlights, you need retroreflective material to actively react, rather than passively reflect. 

[Possibly related: visibility at night, using electroluminescent wire]