British Pendulum Tester: Skid Resistance and BPN Values

The British pendulum tester is a portable device that rates how slippery a surface is. A spring-loaded rubber slider on the end of a pendulum arm swings down and wipes across a wetted patch of pavement. The more friction, the less the arm swings up on the far side. A pointer records the result as a British Pendulum Number, or BPN.

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Where it is used

Spot checks on roads, especially at bends, crossings and approaches.
Road markings, manhole lids, bridge joints and other small areas.
Lab testing of polished aggregate samples.
Walkway and floor slip testing, using a different slider and method.

How the test runs (ASTM E303)

Level the tester over the test spot and set the slider so it contacts the surface over the specified sliding length (about 124 to 127 mm on pavement).
Wet the surface and the slider with clean water.
Release the arm from horizontal. It swings down, the slider wipes the surface, and the pointer stops at the top of the follow-through.
Read the BPN from the scale. Repeat several swings and average them, discarding the first few if the procedure calls for it.
Record the surface temperature, because rubber resilience changes with temperature and corrections may be applied.

What the number means

A higher BPN means more friction. Results mainly reflect microtexture, the small-scale grittiness of the exposed stone, because the slider moves slowly. High-speed skid resistance also depends on macrotexture, the larger channels between stones that drain water. That is why many surveys pair the pendulum with the sand patch test or a laser texture measurement.

Example (made-up numbers): five swings on a worn wheel path read 52, 50, 51, 50 and 51, averaging 51 BPN. A spot between the wheel paths reads 63. The difference shows the polishing caused by traffic.

What lowers BPN over time

Polishing of soft limestone aggregates under traffic.
Bleeding, where binder rises to the surface and covers the stone.
Contamination from oil, rubber and fine dust.
Smooth markings such as worn paint or thermoplastic without beads or grit.

How to raise friction

Choose polish-resistant aggregate in the surface course.
Use open-textured surfaces like an open-graded friction course.
At problem spots, apply a high friction surface treatment with calcined bauxite.
Specify skid-resistant marking materials; see thermoplastic road marking.

Limits

The pendulum tests a small patch, so many readings are needed to describe a road. Agencies with large networks rely on towed or vehicle-mounted friction testers and use the pendulum for spot checks and research. For your own paving quantities, use the asphalt tonnage calculator.

Questions people ask

What does a British pendulum tester measure?

Surface friction at low slip speed. A rubber slider on a swinging arm drags across a wetted surface, and the energy it loses shows up as the British Pendulum Number (BPN).

What standard covers the British pendulum tester?

ASTM E303 for pavement surfaces in the United States. Similar methods exist in British and European standards, including versions for floors.

What is a good BPN value?

There is no single limit. Requirements depend on the site and agency, with higher values expected at bends, crossings and approaches. Low numbers indicate a slippery surface.

Why is the surface wetted?

Wet conditions are when friction matters most and drop the most, so testing wet gives a conservative, comparable result.

Does the pendulum measure macrotexture?

Mostly no. It is sensitive to microtexture, the fine harshness of the stone surface. Macrotexture is measured with methods like the sand patch test.

Can the British pendulum be used on road markings?

Yes. It is commonly used to check slip resistance of paint, thermoplastic markings, manhole covers and other small areas where a towed friction tester cannot be used.

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