GIS Road Safety Analysis · E01 Southern Expressway

Road Risk Analysis

A spatial analysis of road safety hazards along the Southern Expressway from Gelanigama to Pinnaduwa using IRI, curvature, and Jenks Natural Breaks classification.

32.1 KM
Total Length
321
Analysis Points
3
Risk Classes
8
Process Steps
View Risk Map Methodology
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About the Route

Southern Expressway

The Southern Expressway (E01) is Sri Lanka's first expressway, connecting the commercial capital Colombo with the southern coastal cities of Galle, Matara, and Hambantota. It was built to reduce travel time, improve road safety, and stimulate economic development in the Southern Province.

This analysis focuses on the segment from Gelanigama interchange to Pinnaduwa interchange — a 32.1 km stretch that includes elevated sections, sharp curves, and varying pavement conditions.

2011
Year Opened
196 KM
Total E01 Length
100 km/h
Speed Limit
4-Lane
Dual Carriageway
Expressway Network Map
E01 Network

Spatial Risk Assessment

Road Risk Map

Risk Map Gelanigama to Pinnaduwa
HIGH RISK
MODERATE
LOW RISK

Risk was computed at 321 points along the centerline at 100 m intervals, combining International Roughness Index (IRI) and road curvature angle. Jenks Natural Breaks method classified results into three zones.

High Risk Zone 20.9% · 6.7 km
Moderate Risk 30.5% · 11.2 km
Low Risk Zone 44.3% · 14.2 km
High Risk Segments
Sharp curves · High IRI · Accident clusters
6.7
km
Moderate Risk Segments
Transition curves · Moderate pavement
11.2
km
Low Risk Segments
Straight sections · Smooth pavement
14.2
km

Zone-Level Analysis

Risk Zones Explained

High Risk Zone
⚠ High Risk

High Risk Zones

Identified near sharp horizontal curves and sections with high IRI values indicating significant pavement deterioration. These areas have the highest probability of accidents.

6.7 km
Length
20.9%
Share
Risk 70-78
Threshold
Moderate Risk Zone
◈ Moderate

Moderate Risk Zones

Found in transition curve sections where road alignment gradually changes. Pavement is in acceptable condition but curvature poses moderate risk for high-speed vehicles.

11.2 km
Length
30.5%
Share
Risk 62-70
Range
Low Risk Zone
✓ Low Risk

Low Risk Zones

Located in straight road sections with smooth pavement. Minimal curvature and low IRI values ensure comfortable, predictable driving conditions throughout these segments.

14.2 km
Length
44.3%
Share
Risk 36-62
Range

GIS Process

Methodology

01

Road Centerline Extraction

Expressway centerline digitized from Google Maps imagery and converted to a GIS-compatible vector layer.

02

100 m Interval Point Generation

Points generated at 100-metre intervals along the centerline using linear referencing tools.

03

Chainage Calculation

Distance-based chainage values computed for each point to enable spatial referencing along the route.

04

Assign IRI via Linear Referencing

International Roughness Index (IRI) values assigned to each point using dynamic segmentation.

05

Road Curvature Calculation

Deflection angles calculated between consecutive 100 m segments to quantify horizontal curvature.

06

Risk Index Calculation

Weighted composite risk index combining IRI (60% weight) and curvature angle (40% weight).

07

Jenks Natural Breaks Classification

Risk scores classified into three categories (Low / Moderate / High) using Jenks optimal method.

08

Accident Hotspot Identification

High-risk segments cross-referenced with recorded accident data to identify spatial hotspot clusters.

Risk Index Formula
Risk = 0.6 × (IRI) + 0.4 × (Angle)
IRI International Roughness Index — pavement roughness measure (m/km)
Angle Deflection angle at each 100m point (degrees, 0–180)
0.6 / 0.4 Weighting factors reflecting relative contribution to risk

Methodology Summary

Step Process Tool / Method
01 Road centerline extraction Google Maps / QGIS digitizing
02 100 m interval point generation Linear referencing
03 Chainage calculation Route measurement tool
04 Assign IRI using linear referencing Dynamic segmentation
05 Calculate road curvature Geometry / deflection angle
06 Risk Index calculation Weighted formula
07 Risk classification using Jenks Natural Breaks (Jenks)
08 Accident hotspot identification Spatial join / cluster analysis
Weight Distribution
IRI — 70%
CURVE — 30%
0.70
IRI Weight
0.30
Curvature Weight
1.00
Total Weight
32.1
Total KM
High Risk 20.9% 6.7 km
Moderate Risk 30.5% 11.2 km
Low Risk 44.3% 14.2 km

Key Findings

Analysis Results

🔴

High Risk — 6.7 km (20.9%)

Located near sharp curves and rough pavement sections. These segments require immediate road safety interventions and increased signage.

🟠

Moderate Risk — 11.2 km (30.5%)

Found in transition curve areas. Moderate pavement roughness combined with changing alignment elevates risk above average.

🟢

Low Risk — 14.2 km (44.3%)

Concentrated in straight alignments with well-maintained pavement. Represents the safest portions of the analyzed corridor.

📍

Accident Hotspots Identified

Cross-referencing risk zones with historical accident data confirmed that 78% of recorded accidents fell within high-risk classified segments.