Slip vs Trip: Two Different Car-Park Hazards
- PotholeExpert
- 1 day ago
- 4 min read
"Slips and trips" get bundled into one line on most hazard registers. For paperwork that is fine. For actually fixing your car park it is a mistake, because a slip and a trip have nothing in common at the surface. One is a friction problem. The other is a level problem. Treat them as the same thing and you spend money on the wrong control and leave the real hazard live.
If you are running a hazard assessment under the Health and Safety at Work Act 2015, separating the two is the single most useful thing you can do on the walk. It tells you not just where the hazard is, but what kind it is — and therefore what the fix has to be.
What causes a slip
A slip is a loss of grip. The foot keeps moving when it should have stopped. On an asphalt car park the usual culprits are:
Polished aggregate — years of tyres and feet wear the stone in the asphalt smooth, and a smooth stone in the wet has almost no grip.
An over-sealed surface — a heavy seal coat fills the surface texture and leaves a glossy film that is slick when wet. More on this below.
Algae and lichen — the green-black film that grows in shaded, damp corners is one of the most slippery surfaces in any NZ lot, and it spreads through winter.
Oil and fuel sheen — drips in regular parking spots and near entries leave a low-friction film.
Ponding — standing water on a worn surface removes what little grip is left.
What causes a trip
A trip is a sudden level change the foot was not expecting. The causes are structural, not chemical:
Edge ravelling — the crumbling, broken edge of a patch or a kerb, where the surface has lost its bound stone and dropped away.
Sunken patches — old repairs that have settled below the surrounding surface, leaving a lip.
Root heave — tree roots lifting the asphalt into a ridge.
Kerb and threshold lips — the step where asphalt meets a kerb, a door sill or a different surface.
Drain covers — grates and covers sitting proud of, or sunk below, the asphalt around them.
Two hazards, two mechanisms, and the controls do not transfer. Grit on a smooth surface does nothing for a 15mm lip. Grinding a lip flush does nothing for algae.
Why a glossy reseal can make slips worse
This one catches people out, because resealing feels like maintenance. A seal coat protects asphalt from water and UV, and a freshly sealed lot looks cared-for. But pile on a heavy, glossy seal and you fill the surface texture that gave the asphalt its grip — and skid resistance drops. People associate gloss with "new and good", so they do not expect the surface to be slick, which makes the slip more likely, not less.
The point is not to skip sealing. It is to specify a finish that keeps texture on a surface people walk on. Where pedestrians cross — entries, the route to the door, accessible bays — you want a textured, non-glossy finish, not a mirror. Tell whoever seals your lot that pedestrian grip matters and the spec changes.
Skid resistance, in plain terms
You do not need lab numbers to manage a car park, but the principle is simple: grip comes from surface texture and clean stone, and it falls away as the surface polishes, films over, or floods. A walked-on surface needs more grip than a parking bay because people change direction, carry loads and move faster than a creeping car. So on a car park, the pedestrian lines deserve the highest-grip surface, not the cheapest finish. If a reseal has left a walked route glossy, that is a slip control failure even if the asphalt is structurally perfect.
Match the control to the cause
Once you have labelled each hazard, the fix follows directly.
For a slip, restore friction. Clean off algae and oil, and keep them off. Re-texture or re-spec an over-sealed surface. Fix the drainage so the area stops ponding.
For a trip, fix the level. Grinding a lip is at best temporary; the durable answer is to saw-cut back to sound asphalt, relay and compact to the surrounding level, and seal the joints so the edge stays put. A feathered cold patch over a sunken repair just creates the next lip. The same goes for any pothole on a pedestrian line — cut it back, fill, compact and seal, so it does not return each winter as both a trip and a ponding point.
Some defects are both. A sunken drain cover trips a foot and ponds water that then makes the area slippery — so it needs the level fixed and the drainage sorted.
A combined slip-and-trip walk-through
Run one walk, two columns. In the wet if you can, because slips only show themselves when the surface is wet.
Walk the lines people actually use — bay to door, accessible bay to entrance, trolley route.
At each spot, ask which it is: would I slip here, or trip here?
Slip column: note polished, glossy, algae-covered, oily or ponding areas.
Trip column: note ravelled edges, sunken patches, root heave, kerb lips, proud or sunken covers.
Flag anything that is both.
Rank by where people walk most, not by size.
How a fix gets booked
Photograph what you find — a wide shot and a close-up of each spot, taken in the wet for the slip hazards — and send them through. You get a fixed quote within 24 hours, with the work booked inside 48 once you accept. For a live lot we run cones and a spotter, or work after-hours, so the car park stays open. Every repair carries a 12-month workmanship warranty and a dated before/after photo report for your records.
Do the wet walk, label each hazard as slip or trip, and get a fixed quote. Naming the hazard correctly is what gets it fixed properly.
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