Pavement Roughness vs Texture: Why Pavement Smoothness Doesn’t Always Equal Safety

Introduction

Pavement condition assessment frequently employs the International Roughness Index (IRI) as a primary quantitative metric for longitudinal profile regularity. While IRI effectively characterises ride quality, user comfort, and network-level serviceability, its singular application provides an incomplete evaluation of surface performance, particularly in terms of safety. Pavement surface friction, a critical parameter governing vehicle-pavement interaction, is a function of both macrotexture and microtexture. A smooth ride doesn’t necessarily mean a safe one; beneath the tyres, the story gets more complex.

To fully understand road safety and performance, we must look beyond IRI and assess the pavement’s surface friction properties. While often correlated with roughness, these surface textures can tell a very different story, especially when they diverge.

Defining Technical Terms

• Roughness (IRI) measures the vertical deviation of a road surface over long wavelengths, typically greater than 0.5 m. It reflects how a vehicle dynamically responds to these surface variations, impacting ride comfort, fuel efficiency, and suspension wear. Think of it as the big bumps and dips you feel while driving — not just what you see.

• Macrotexture is the coarse-scale surface texture with wavelengths between 0.5 mm and 50 mm and peak-to-peak amplitudes typically ranging from 0.01 mm to 20 mm. It is shaped by the arrangement and size of aggregates in the surface layer and strongly influences water drainage and high-speed skid resistance. Check out this post to study more about macrotexture.

• Microtexture refers to the fine-scale roughness on the surface of individual aggregate particles. With wavelengths less than 0.5 mm, it plays a critical role in tyre grip at low speeds, particularly in dry or lightly wet conditions. It enables the tyre rubber to interlock with microscopic surface features, generating the friction needed for safe braking and handling.

These three indicators are measured at different scales, but they all contribute to overall pavement performance. Understanding their interplay is key to diagnosing both functional and structural issues.

Comparison Table

Roughness vs Macrotexture vs Microtexture

Correlation Between Roughness and Texture

In many flexible and rigid pavements, a positive correlation is frequently observed between increasing roughness (higher IRI values) and the degradation of surface texture (reduction in macro- and microtexture). Common deterioration mechanisms, including traffic-induced wear, environmental degradation (oxidation, thermal cycling), and material aging, often concurrently impact both the pavement's longitudinal profile and its frictional characteristics. Aggregate polishing due to traffic abrasion reduces microtexture, while surface deformation and material loss contribute to increased roughness and potentially diminished macrotexture. In such cases, a high IRI often pairs with low MPD (Mean Profile Depth) and poor friction test results—indicating a pavement that is both rough and unsafe.

However, this relationship doesn’t always hold, lets consider deviations that can occour.

Deviations from Correlation

Consider the four pavement condition scenarios

Source: Lars Forslöf

• Case 1: Bad Roughness + Bad Texture
  This is a worst-case scenario. The ride is bumpy, and the pavement lacks micro and macrotexture, making it unsafe and uncomfortable.

• Case 2: Good Roughness + Bad Texture
  It is often found on ravelling pavements or those with surface loss. The road feels smooth, but the microtexture is compromised, creating a risk of skidding.

• Case 3: Good Roughness + Good Texture
  The ideal scenario. Smooth ride, good grip, well-drained. Maintenance is preventive, not reactive.

• Case 4: Bad Roughness + Good Texture
  A deceiving surface. It appears black and smooth, but hidden geometry causes a high IRI. Despite good friction, the ride is uncomfortable.

 Why This Matters

Both texture and roughness affect safety, but in different ways:

• Microtexture ensures contact between the tyre rubber and the road, which is critical for both dry and low-speed wet friction.

• Macrotexture helps disperse water under high-speed conditions, thereby mitigating hydroplaning potential and enhancing wet skid resistance.

• Roughness influences comfort, fuel economy, and structural condition, but does not directly indicate skid safety.

A pavement exhibiting high IRI values may still possess adequate frictional properties, while a pavement with low IRI values can present a significant skidding hazard due to inadequate surface texture. This is particularly relevant in evaluating granular surfaced roads and asphalt pavements with polished aggregate.

Engineering Actions and Interventions

When both roughness and texture are poor, full resurfacing or rehabilitation may be required. However, when texture alone is the issue, targeted surface treatments like resealing, grooving, or skid-resistant overlays may restore friction without structural repairs.

Poor microtexture on gravel roads may limit speeds to under 30 km/h even if the alignment is sound. In these cases, reconstruction, crushing of coarse material, and reshaping/grading are necessary to restore safe drivability.

The takeaway? Always measure the right thing for the right purpose. Relying on IRI alone may miss critical safety risks that are hidden in the texture profile.

Conclusion

Smooth doesn’t always mean safe. To deliver pavements that are truly fit for purpose, engineers must evaluate both ride quality and surface friction. By integrating assessments of roughness (IRI), macrotexture (MPD), and microtexture, we gain a comprehensive understanding of pavement performance and risk.

Only then can we ensure roads are not just drivable, but dependable.