This document discusses research analyzing the crash risks for heavy trucks in horizontal road curves. It finds that current road design does not adequately consider the dynamics of large vehicles, underestimating the superelevation needed to prevent rollovers and skidding. Using a more complex vehicle model that accounts for factors like center of gravity height and weight transfer, the analysis determines substantially more banking is required than traditional models to reduce heavy truck crash risks in curves. Further research is still needed to better understand issues like the effects of downhill grades and variable friction conditions.

1. LOWERED CRASH RISK
WITH BANKED CURVES
DESIGNED FOR HEAVY TRUCKS
International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014

2. LOWERED CRASH RISK
WITH BANKED CURVES
DESIGNED FOR HEAVY TRUCKS
Presenting author:
MSc (Civ Eng) Johan Granlund, CTO at WSP Group, Sweden
International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014
Co-authors:
BSc (Civ Eng) Irene Haakanes, Asplan Viak AS, Norway
BSc (Civ Eng) Razia Ibrahim, Norwegian Public Roads Administration

3. OVERVIEW
Road Designers Unaware of HGV Dynamics. Scope: Is Current Design of Horizontal Curves Good Enough? Existing Knowledge on Crash Risks in Curves. Traditional Calculation of Superelevation Demand due to Skid only. Analysing Cornering Forces With a More Complex Vehicle Model.
•
Influence of CoG Position on Rollover.
•
Influence of Weight Transfer and Split Friction on Skidding. Results from the New Analysis: More Superelevation is Required! Examples of Need for More Research.
Granlund et al. Lowered Crash Risk with Banked Curves Designed for Heavy Trucks.
International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014

4. ROAD DESIGNERS ARE
UNAWARE OF HGV DYNAMICS
Outercurves are banked into superelevation in order to reduce crash risk due to high demand for side friction between tyres and road.
Road design codes world wide include analysis of cornering forces acting on a point-mass model of a car.
Granlund et al. Lowered Crash Risk with Banked Curves Designed for Heavy Trucks.
International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014
Unlike what is shown in the model, the Centre-of-Gravity (CoG) is in traditional analysis assumed to be located at the tyre footprint. In fact, the “CoG height above pavement” is not even a parameter! -How reasonable is this for heavy vehicles with high CoG?

5. IS CURRENT CURVE DESIGN GOOD ENOUGH
FOR SAFE HGV OPERATION?
The scope for this work: “Investigate the relevance of calculating the superelevation demanded by heavy vehicles with high CoG, instead of superelevation demanded by low passenger cars.”
Granlund et al. Lowered Crash Risk with Banked Curves Designed for Heavy Trucks. International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014
The accident mechanism in focus is not only skidding (as in traditional curve design); also rollover is analyzed here.

6. EXISTING KNOWLEDGE (1):
HIGH COG & FLAT CURVES = HAZARD
Crash rates 2 - 4.5 times higher in curves. Leonard et al., 1994.
Overrepresented crashes in curves are single vehicle crashes, rollovers, collisions, night crashes and crashes involving drivers under the influence of alcohol and drugs. Elvik et al, 2014.
Heavy trucks show the highest raise in crash rates between straight and curved road sections. “Flat curves” worst. Haywood, 1980.
Less than 4 % of crashes with HGVs are rollovers. Winkler, 2000.
50 % of crashes where HGV occupant died were rollovers. NVF, 2011.
Largest risk factors for rollover are high CoG, high speed, cargo displacement, bad road conditions, driver behaviour (incl. road rage as well as lack of focus on the driving task and the payload) and secondary faults, such as skidding into a curb or low crash barrier causing a “trip and fall” rollover. TYA, 2014.
Granlund et al. Lowered Crash Risk with Banked Curves Designed for Heavy Trucks.
International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014

7. EXISTING KNOWLEDGE (2): MORE SUPERELEVATION IS SAFER
Outer-curves 5 times as dangerous as inner-curves. Eliminating over- risk at outer-curves would prevent 10 % of all fatal road traffic crashes in Sweden. Lindholm, 2002.
Many outer-curves have insufficient banking (adverse camber) with respect to speed limit and slippery surface conditions. Old curves were constructed for low speed wagons pulled by a donkey, but do not meet the needs of motorized highway vehicles. EU ROADEX III, 2008.
1 % increased super-elevation results in 5 % reduction in heavy vehicle loss-of-control crash risk while cornering. De Pont & Milliken, 2005.
Split friction in curves is extremely hazardous. NVF, 2011.
Granlund et al. Lowered Crash Risk with Banked Curves Designed for Heavy Trucks. International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014

8. ANALYSING CORNERING FORCES: RESULTS FROM TRADITIONAL MODEL
Demanded superelevation tan(θ) [%] at given speed and curve radius is derived by inserting a selected side friction supply factor μs.
Granlund et al. Lowered Crash Risk with Banked Curves Designed for Heavy Trucks.
International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014
μs
Source: NCHRP 439
Model only for skid analysis.

9. RESULTS FROM MORE COMPLEX MODEL:
ROLLOVER MODE ANALYSIS - GENERAL
Three analyzed application cases:
•
The Roundabout / The Bad Highway Ramp.
•
The Modern Highway Curve.
•
Old Road Outer-Curve.
Granlund et al. Lowered Crash Risk with Banked Curves Designed for Heavy Trucks. International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014

10. RESULTS FROM MORE COMPLEX MODEL:
ROLLOVER MODE ANALYSIS (1)
Insert g = 9.81 m/s2 and
track width (centre) b = 2.20 m.
Granlund et al. Lowered Crash Risk with Banked Curves Designed for Heavy Trucks.
International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014
Case A. The Flat Roundabout / The Bad Highway Ramp:
Set curve radius R = 20 m and crossfall θ = 0.
Assume CoG lateral displacement d = 0.3 m
Set speed v = 43 km/h (12 m/s).
The equation leads to a rollover height of h = 1.9 m.
h = 1.9 m indicates a rollover risk, since being lower than CoG-heights expected for dual floor trailers with several common load combinations (up to h = 2.3 m). Dahlberg, 1999.

11. RESULTS FROM MORE COMPLEX MODEL: ROLLOVER MODE ANALYSIS (2)
Insert g = 9.81 m/s2 and
track width (centre) b = 2.20 m.
Granlund et al. Lowered Crash Risk with Banked Curves Designed for Heavy Trucks. International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014
Case B. The Modern (but here Flat) Highway Curve Our curve has large radius R = 500 m and crossfall θ = 0. Set speed v = 80 km/h (22 m/s). Assume CoG lateral displacement d = 0.3 m The equation leads to a rollover height of h = 14 m. h = 14 m shows a large margin to rollover, since being multiple times higher than real Heavy Goods Vehicle CoG-heights.

12. RESULTS FROM MORE COMPLEX MODEL:
ROLLOVER MODE ANALYSIS (3)
Insert g = 9.81 m/s2 and
track width (centre) b = 2.20 m.
Granlund et al. Lowered Crash Risk with Banked Curves Designed for Heavy Trucks.
International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014
Case C. The Old Road Outer-Curve
Our curve has tight radius R = 125 m. Set speed v = 80 km/h (22 m/s).
Assume CoG lateral displacement d = 0.3 m.
tan(θ)
Resulting hrollover [m]
+0.075
4.5
+0.05
4.1
+0.025
3.8
0.00
3.55
-0.025
3.3
-0.05
3.1
-0.075
2.9
Flat or negative crossfall reduce the rollover risk margin significantly. Margin is needed for outer factors such as loads from windburst, road unevenness et c.

13. RESULTS OF THE SKID MODE ANALYSIS
The paper exemplifies a dangerous case with a flat sharp curve on a slick road where the side friction in the outer wheelpath is lower than in the inner wheelpath (μ2 < μ1), where highway speed cause outside weight transfer (N2 > N1). The complex model shows a demand of superelevation tan(θ) = 6.2 %, while the basic model demands only tan(θ) = 2.6 %. The relative difference was 138 % higher superelevation. Clearly the basic model significantly underestimates the need for crossfall / superelevation / banking.
Granlund et al. Lowered Crash Risk with Banked Curves Designed for Heavy Trucks. International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014
From the more complex truck model
From the basic point mass model

14. NEED FOR FURTHER RESEARCH
Granlund et al. Lowered Crash Risk with Banked Curves Designed for Heavy Trucks.
International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014
More knowledge is needed about:
•
Superelevation demand in curves at downhill grades.
•
Side friction factors - not to be confused with the twice as high brake friction factors - between truck tyres and road surfaces. (This goes for both summer tyres as well as for softer winter tyres) Particular focus should be given - both in research and in daily road management - to actions aiming to prevent or at least reduce split friction supply between outer and inner wheelpath in horizontal curves during various vehicle operation conditions (road surface contaminations, et c).

15. SUMMARY
Road Designers Unaware of HGV Dynamics. Scope: Is Current Design of Horizontal Curves Good Enough? Existing Knowledge on Crash Risks in Curves. Traditional Calculation of Superelevation Demand due to Skidding. Analysing Cornering Forces With a More Complex Vehicle Model.
•
Influence of CoG Position on Rollover.
•
Influence of Weight Transfer and Split Friction on Skidding. Results from the New Analysis: More Superelevation is Required! Need for More Research, f x joint influence of downhill grades.
Granlund et al. Lowered Crash Risk with Banked Curves Designed for Heavy Trucks. International Heavy Vehicle Transport Technology Symposia HVTT13, San Luis, Argentina 2014