Slopes, surfaces, bends, and calming features effect on speed.
Terrain can be your ally. Catalogue calming features, kerbs, furniture, surfaces, gradients, and bends that naturally reduce run-up speed or enforce vectors. Note seasonal changes (rain, sand) and temporary measures for events (239, 825). Use the evidence template to justify lower or higher HVM bollard ratings or crash rated bollard densities in selection (432–435) and arrays (321–326). Include one-sentence context that naturally links upward to the parent hubs (this section and the chapter hub). Add SIRA context with a link to SIRA Bollards (UAE) when relevant. Link installation pages only if helpful: What to Expect and Installation Guide.
Important: This is a general guide. For live projects we develop a tailored
Method Statement & Risk Assessment (MS/RA) and align with authority approvals (e.g., SIRA) where in scope.
227.1 Speed humps/rumble strips
Calming reduces achievable speeds along vectors (222). Use existing humps to lower HVM bollard counts. Still keep the chosen crash rated bollard within certified speed bands (413).
Speed humps, cushions, and traffic calming textures increase delays and reduce the effective run-up distance, which depresses credible impact speed. In a VDA, note exact positions, dimensions and spacing; combine with speed estimation methods (224) to quantify reductions.
Do not over-credit calming: humps may be bypassed by service lanes, or hit obliquely. Validate with timed traverses and peak/off-peak samples, then reflect any residual uncertainty in sensitivity & safety factors (228). If calming is temporary or removable, model the worst credible layout.
| Aspect | What matters | Where to verify |
|---|---|---|
| Geometry | Height, ramp length, spacing | Speed Methods (224) |
| Bypass risk | Alternate paths without calming | Approach Paths (214) |
227.2 Kerbs & bollard lines
Kerb height/shape and prior lines can trip wheels and redirect angles (324). Align new HVM bollard posts to exploit these effects while retaining compliant clear gaps (232). Confirm crash rated bollard head shape suits kerb proximity (313).
Kerb upstands and back-of-kerb offsets can destabilize light vehicles at glancing angles, nudging impacts towards the array’s strongest axis. Coordinate array position with corners & pinch points (324), maintaining the clear-gap rule (232) for pedestrians, wheelchairs and prams.
Where kerbs are flush or ramped, don’t assume redirection. Check bollard head profiles and attachments (313) for snag/capture height and sightline impacts. Capture photos and scaled sketches on overlays (936).
227.3 Street furniture influence
Benches, planters, and shelters can shield or create ramps. Keep furniture outside main vectors (214) and use it to lengthen run-up pathing for HVM bollard advantage; ensure it doesn’t invalidate the crash rated bollard orientation (421).
Furniture clusters can force gentle chicanes that reduce speed before the array; equally, low planters can become launch ramps. Map furniture against approach vectors (214) and add or relocate items to extend the effective run-up without breaching accessibility or fire clearances.
Always check rating-critical dependencies (421) so furniture placement doesn’t conflict with certified orientations, set-backs, or capture heights. For door lines, compare with door protection arrays (323).
227.4 Surface type & friction
Pavers vs asphalt vs polished concrete change acceleration (224). Higher μ lets you reduce HVM bollard density cautiously; never claim a crash rated bollard rating above its certificate.
Record the surface friction coefficient by surface type, wear, and contamination. In the segment model, higher μ increases achievable speed for a given segment model (accel)—so reductions in bollard count must be justified with evidence and safety factors (228).
Conversely, very smooth tiles or polished concrete may reduce grip under wet conditions, changing braking and steering near the array. Where resurfacing is planned, run both “current” and “future surface” scenarios in the VDA.
227.5 Gradients and cambers
Downhill stretches increase speed; crossfall affects stability. Tilt HVM bollard patterns to counter skews (321). Review crash rated bollard photos for similar grade conditions (431).
Measure longitudinal gradient and crossfall (camber). Downhill segments increase potential energy conversion into speed; crossfall can pull vehicles toward or away from the array. Consider slight staggering or array pattern (321) rotation to align with governing vectors and reduce glancing bypass risk.
Where grades are significant, look for certified installations on similar slopes in manufacturer evidence or certificates (431). If none exist, be conservative in spacing and model uncertainty explicitly.
227.6 Bends and sightlines
Tight bends cut speed but may hide arrays (237). Use staggered HVM bollard layouts (321.2) and conspicuity cues (357). Keep crash rated bollard finishes durable to cleaning/UV (366, 362).
Curvature often reduces speed organically, but short-radius bends may obscure arrays and signage. Combine bend-induced slowing with staggered layouts (321) and strong signage & markings (357) to preserve legibility for legitimate users.
On coastal or dusty routes, select finishes that withstand frequent washing and high UV—see coatings (362) and aesthetic finishes (366). Where approvals apply, align any conspicuity treatments with local authority guidance.
227.7 Seasonal effects (rain/sand)
Water and sand lower μ unpredictably. Add safety margin to HVM bollard spacing (232) and choose a crash rated bollard enclosure/coating set for dust/salt (516, 361).
Rain, dew, and wind-blown sand change friction, braking and steering near impact points. In the VDA, include wet and sanded cases and reflect the range with safety factors (228). For equipment, match enclosure protection (516) to site exposure and specify materials from materials selection (361).
Where seasonal street sweeping or de-sand schedules exist, note them in operational assumptions; if absent, design for the “dirty worst month.”
227.8 Temporary measures
Event chicanes and cones modulate risk (239). Ensure temporary HVM bollard gaps don’t exceed acceptance (232). Do not claim temporary fixes as crash rated bollard performance.
For event mode (825), portable barriers, cones and tape can lengthen the path and slow vehicles, but they are easily altered. Treat them as management measures, not structural mitigation. Recheck gap acceptance (232) after crowd-control equipment is placed.
Document the temporary layout, times of use, and stewarded gaps. If gaps must open for emergency access, define the override modes (354) and responsibilities clearly.
227.9 Evidence template
Tabulate feature → speed impact → design response (912–913). This supports HVM bollard decisions and documents crash rated bollard sufficiency (938).
Use the Site Assessment Template (912) and the VDA Worksheet (913) to record each terrain/calming feature with location, measurements, observed timings, and photos. Add a column for “design response” (spacing, array pattern, product rating) and a final column for “assumptions & uncertainty.”
Attach the table and overlays to the submission-pack guidance (938), ensuring reviewers can reproduce results. Cross-reference any dependencies or limitations in the chosen product’s certificate set (431).
Related
External resources
- NPSA — Hostile Vehicle Mitigation (HVM) guidance
- ASTM F2656 — Vehicle impact performance of perimeter barriers
- PAS 170-1 — Low-speed impact testing for bollards
227 Terrain & Calming Effects — FAQ
How much can speed humps actually reduce impact speed in a VDA?
It depends on hump geometry, spacing, and whether drivers can bypass them. Use timed runs and surface notes to quantify and then apply sensitivity factors (228). Never assume a fixed reduction without evidence.
Can we rely on street furniture instead of more bollards?
Furniture can lengthen paths and encourage lower speeds, but it is not a certified barrier. Use it to shape vectors, while keeping arrays and gaps within rules (232) and preserving product dependencies (421).
Do downhill slopes always mean we need a higher rating?
Not always, but downhill segments usually increase achievable speed. Model gradient explicitly and, if uncertainty remains, use safety factors (228) or tighter spacing rather than inflating ratings without basis.
Are temporary event chicanes acceptable evidence for lower ratings?
No. Treat temporary chicanes as management controls. They may justify temporary spacing changes, but they cannot replace certified crash-rated performance or be used to claim a higher standard.