A PRIORITISATION SCHEME FOR THE SAFETY MANAGEMENT OF CURVES Presented by: Neil Jamieson Research Leader, Tyre-Road Interactions Opus Central Laboratories

2 Why focus on curves?

3 Loss of control on curves are the largest cause of injury crashes on NZ rural State Highways! In 2009: Amounted to 1309 reported injury crashes Corresponds to: – 49% of reported injury crashes on rural SH’s – 36% of all reported injury crashes 1210 (92%) occurred on moderate or easy curves 471 (36%) occurred in wet Answer

4 For a curve defined as: Collective risk = Fatal Crashes + Serious & Minor Injury Crashes on Curve Number of Years of Data Personal risk or crash rate is a measure of the likelihood of an individual road user being involved in a crash as they enter a curve i.e. Personal Risk = Fatal Crashes + Serious & Minor Injury Crashes on Curve (No. of years of data × 365 days × AADT)/10 8 Collective and personal risk metrics

5 Curve radius & collective risk

6 Curve radius & personal risk

7 Relied on T10 specification, which aimed to equalise personal risk across SH network through investigatory skid resistance levels (IL’s). Prior to October 2010, curves < 250mR were managed to a skid resistance level that was 25% greater than for all other curves on SH network (IL=0.5 c.f. IL=0.4). Curves ≥ 250mR (85 km/h curves) treated the same as straights (event free). Too simplistic for “safe system approach”! Previous safety management of curves

8 Potential for reducing SH crash numbers

9 1.All curves < 400mR identified 2.Crash rate calculated using a predictive model which has as inputs: – curve speed (derived from geometry) – curve length – approach gradient (averaged over 100 m prior to curve) – difference between approach speed and curve speed 3.Risk ranking of “high”, “medium” or “low” assigned to each curve on basis of predicted crash rate. Slides which follow expand on the above three steps Solution for more effective safety management

10 Locating start of curve Typical Right Hand Curve Straight Spiral Circular Arc Tangent Point CS Start of curve  “Point where radius < 800m”

11 Estimation of curve radius Typical Right Hand Curve Straight Spiral Circular Arc Tangent Point CS Superelevation  (crossfall)  Averaged over tightest 30mR Curve Radius  Averaged over tightest 30m of the curve Curve included if <400mR

12 Locating end of curve Typical Right Hand Curve Straight Spiral Circular Arc Tangent Point CS End of curve  Radius > 800m

13 Curve Crash Rate = (10 8 ⁄365)×L 1 ×exp(L 2 ) L1 & L2 are linear combinations of transforms of road characteristics as follows: L1:a constant square root of curve length L2:OOCC (i.e. difference between approach & curve speeds) curve speed skid resistance approach gradient log 10 (ADT) year NZTA administration region Poisson linear/log-linear model

14 Predicted effects on curve crash rates - ADT

15 Predicted effects on curve crash rates - SCRIM

16 Predicted effects on curve crash rates – curve length

17 Predicted effects on curve crash rates – approach gradient

18 Predicted effects on curve crash rates – speed difference

19 Observed & modelled crash numbers

20 Predicted crash rate distribution

21 Predicted Crash Rate (crashes per 100 million vehicle entering curve) Curve Risk RatingSCRIM Investigatory Level PCR > 14High0.55 7≤PCR≤14Medium0.50 PCR < 7, R<250mLow (Cat 2)0.45 PCR < 7, 250m≤R≤400mLow (Cat 4)0.40 Default risk ratings of curves and IL’s

22 High risk curves >250mR lowered to low if speed difference less than 15km/h High risk curves >250mR lowered to medium if speed difference below 20km/h Medium risk curves (<250mR) raised to high if speed difference greater than 35km/h High risk curves <250mR lowered to medium risk if speed difference <20km/h Moderations to default curve risk ratings

23 Actual injury crash rates versus risk rating

24 Superseded T10:2002 – curves (<250mR) – Approximately 1041 km’s (9.3% of network), IL=0.5 T10:2010 (curve risk rating incorporated) – ≈ curves (≤400mR) – Equates to 2620 km’s (23.4% of network) 505 km (4.5% of network) low risk (IL=0.40 or 0.45) 1365 km (12.2% of network) medium risk (IL=0.50) 750 km (6.7% of network) high risk (IL=0.55) Implications for NZ’s rural SH network

25 Extending <250mR curves (T10:2002 site cat 2 curves) to include transition spiral increases length of network managed to an IL=0.5 from 1041 kms (9.3% of network) to 1699 kms (15.6% of network). However, B/C ≈ 10. Applying curve risk rating procedure to extended curves gives B/C≈ 26. Targeted skid resistance management of curves seen as a very cost-effective safety measure. Curve table incorporated in RAMM to assist industry. Concluding Remarks