The types of hair are pretty easy to tell apart, not due to width measurements, but due mostly to the cross section shape and pigment granule distribution.
is thin and almost flat in cross section (tape like), with a tendency to very tight curls. Sometimes it grows in tiny clumps, called peppercorn hair. African hair grows the slowest, at about 0.9cm a day. It’s angle of growth is very small, nearly parallel to the scalp. In colour, it is nearly always black in Africans. The only time you’ll see negroid hair of a different colour is if the individual is an albino, or has European ancestry, or if they dye it. Naturally coloured African hair has densely distributed pigment granules (hair shaft may be opaque) that are arranged in prominent clumps. The hairs also twist irregularly about their longitudinal axis, it has been described as a twisted oval rod.
is a slightly irregular oval shape in cross section. It can to ruler straight, curly as an Africans, and every degree of curl between. It has the widest range of colours; black, auburn, shades of fair and brown to a near white blond. The hair grows out of the skull at an oblique angle, at a rate of about 1.2cm a month. the pigment granules are sparse to moderately dense with fairly even distribution .
grows the fastest at an average of 1.3cm a month. Its is more circular in cross section, although not perfectly regular. It grows out of the scalp at a right angle. The pigment granules are densely distributed and often arranged in large patchy areas or streaks. it is nearly always black.
Various links on racial variations of hair.
All three hair types under an electron microscope. The shape difference between African and European hair doesn’t show well here though.
An interesting exception to the Asian hair average is Ainu hair, which I have seen described as Caucasian in cross section. it’s also reputed to come in dark brown and a reddish brown colour.