Why bikers avoid white lines: the truth about road markings

“Reflective road markings make all motorists safer”. Well, not quite all motorists…

You can dismiss the humble white line as just ‘paint’, but those in the trade know it as a much more high-tech invention called the Thermoplastic Road Marking or ‘TRM’.

TRMs were originally developed during the 1940s, their inbuilt reflectivity helping motorists navigate darkened roads during wartime blackouts. Since doing their bit for their country, they’ve been universally accepted into the pantheon of Jolly Good Ideas and now account for around 95% of road markings in everyday use.

TRMs have certainly done much to make modern roads safer, but motorcyclists have always had a love-hate relationship with them. Here’s why:

Skid Resistance

Common sense suggests that road markings are slippery and waiting to catch you out. But are they?

Science can help answer this. The skid resistance of a road marking can be accurately measured with a clever piece of apparatus called a British Pendulum. Originally designed in the 1940s, the UK Transport Research Laboratory developed it as an accurate way of measuring the ‘slipperiness’ of road surfaces. It works by swinging a rubber heel lightly across a surface and measuring the resulting resistance encountered as a Coefficient of Friction. Pendulum testers are accurate, simple and compact, so much so that the legal community are increasingly using them to help scientifically decide the truthfulness of ‘slip and trip’ injury claims.

skid_test
Measuring the anti-skid properties of a road marking using a British Pendulum

In the UK, a road marking’s skid resistance is measured to produce a pendulum test value of 0 to 65, where 65 is the higher, grippier measurement and 45 is considered a minimum acceptable value. Not surprisingly road surfaces exhibit similar frictional values, beginning at 45 and rising to 68 and above, depending on the type of aggregate used and the wear condition of the surface.  What this means is that on a dry road grip remains measurably even across the carriageway. Good for motorcyclists. Good for everyone.

But in the rain, as all motorcyclists know, things deteriorate rapidly.

Retro-reflectivity

Retro-reflectivity is the ability of something to reflect light back to its source. It’s a safety requirement of all road markings which people see as a good thing. And it is – mostly.

delta retrometer
Using a retrometer to measure retro-reflectivity

Retro-reflectivity was first experimented with in the United States during the 1930s when the Minnesota Mining and Manufacturing Company (or 3M as you might know them) took glass beads normally applied to the ‘silver screens’ of cinemas and sprinkled them on wet road paint.

reflective glass beads
Waiting to guide your way

The result was a simple method of making paint reflective and the principle survives to this day.

Of course every great idea comes with a downside and this otherwise dazzling safety invention’s significant one is that wet glass beads and motorcycle tyres are not the natural bedfellows we’d like them to be. Put simply, asphalt’s rough surface and better ability to drain water than glass bead covered TRM means white lines are indeed slippier in the wet than the surrounding road surface.

Modern TRM includes rough aggregates to try to balance the glass beads’ inherent slipperiness, but the fact is that the more reflective beads added to the surface layer of a road marking, the lower its skid resistance becomes.

Highways contractors, their hearts ever in the right place, frequently specify more reflective beads on road sections considered dangerous. The irony is that once it rains, what’s considered safer in one regard becomes (for the motorcyclist at least) more hazardous in another.

Bikers more than anyone need to remember reflectivity comes at the price of grip. Follow the line by all means. Just don’t brake hard or corner on it.

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