Why Road Geometry Drives Single‑Vehicle Crashes: Insights from Haworth (2015)

MotoScience | Research‑Backed Riding Insight
Study referenced:

Characteristics of road factors in multi and single vehicle motorcycle crashes in Queensland (Narelle Haworth, 2015, Centre for Accident Research & Road Safety – Qld (CARRS-Q) Faculty of Health; Institute of Health and Biomedical Innovation

Purpose of the Study

Motorcyclists accounted for 6.4% of all police‑reported crashes and 12.5% of fatal crashes in Queensland between 2004 and 2011. Within this, 43% were single‑vehicle (SV) crashes and 57% were multi‑vehicle (MV) crashes.

Although overall motorcycle crashes declined during the study period, this masked a crucial divergence: SV crashes increased while MV crashes decreased.

Haworth’s study set out to understand:

• • how SV and MV crashes differ,
  • which road‑environment factors predict each type, and
  • why the two crash types are following opposite long‑term trends.

The analysis used descriptive comparisons and regression modelling to examine the influence of road geometry (horizontal and vertical alignment) and surface condition (sealed/unsealed, wet/dry) on crash occurrence.

Key Findings

1. Single‑vehicle and multi‑vehicle crashes follow different trends

Across the 2004–2011 period:

• • Single‑vehicle crashes increased, despite overall crash reductions
  • Multi‑vehicle crashes decreased

This indicates that the two crash types are driven by different mechanisms and should not be treated as a single category.

2. Road geometry is a major predictor of single‑vehicle crashes

The regression models showed that SV crashes were strongly associated with:

• • Tight or complex horizontal curves
  • Significant vertical alignment changes (crests, dips)
  • Combinations of both (crest‑into‑bend, dip‑into‑bend)

These features increase the perceptual and control demands placed on riders, particularly in terms of:

• • preview
  • speed planning
  • lean‑angle judgement
  • grip management

These geometric factors had much weaker effects on MV crashes.

3. Surface condition matters more for single‑vehicle crashes

SV crashes were more likely on:

• • wet surfaces
  • unsealed surfaces
  • surface transitions

This reinforces that SV crashes are sensitive to traction‑related errors and rider‑road interaction.

MV crashes showed little sensitivity to these factors.

4. Multi‑vehicle crashes are dominated by traffic interactions

MV crashes were more strongly associated with:

• • intersections
  • turning movements
  • right‑of‑way conflicts
  • visibility and expectation failures by other drivers

Road geometry and surface condition played a comparatively minor role.

Implication for Motorcyclists: single vehicle and multi-vehicle crashes happen in different ways

The study reinforces a critical distinction:

1. Single‑vehicle crashes are “road‑demand failures”

They occur where the road environment exceeds the rider’s available capacity at that moment. Riders are most vulnerable when:

• • preview is restricted
  • geometry changes rapidly
  • vertical alignment hides what’s coming
  • surface grip is reduced or unpredictable

These are perceptual‑cognitive challenges, not simply “going too fast”.

2. Multi‑vehicle crashes are “traffic‑interaction failures”

They arise from:

• • being unseen
  • being unexpected
  • being misjudged by other drivers

This is where defensive positioning, conspicuity, and anticipation of right‑of‑way violations matter most.

 

Why this matters for riders

Riders need different strategies for SV vs MV risk. Road design plays a larger role in SV crashes than commonly acknowledged. Training should emphasise perceptual and predictive skills in high‑demand geometry. Practical takeaways for riders include:

• • Improve preview and speed planning on curves.
  • Adjust early for surface transitions.
  • Use defensive positioning to mitigate MV risk.

Conclusion

Motorcycle crashes are often reported as a single category, but Haworth’s analysis of Queensland crash data shows something far more important: single‑vehicle (SV) and multi‑vehicle (MV) crashes not only behave differently and respond to different risk factors, but in this location and in this time period, they are also following different long‑term trends. When two crash types move in opposite directions over the same period, it’s a signal that the underlying mechanisms are being pushed by different forces.

For MotoScience, this distinction is crucial. It helps us separate rider‑road interaction failures from traffic‑interaction failures, and it gives us a clearer view of how road design shapes rider error.