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A QUANTITATIVE MODEL OF ROAD-SURFACE SAFETY. Jiangbi Hu , Transportation Research Center , Department of Civil Engineering,Beijing University of Technology , China 2011.5. Contents. Introduction Safety factors in the interaction between vehicle and pavement

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JiangbiHu,

Transportation Research Center, Department of Civil Engineering,BeijingUniversity of Technology, China

2011.5.

• Introduction

• Safety factors in the interaction between vehicle and pavement

• Pavement performance and traffic safety

• Pavement safety model

• Model validation

• Conclusions

1.Introduction

• Road traffic system is a dynamic and complicated system , which provides safe, comfortable and economic road condition for road customer

• Road(include its structures) condition is an important reason that causes the traffic accident and has caused accidents up to 30% along with driver interaction.

• Driver safety is associated with surface roughness and skid resistance.

• The friction is the basic parameter that limits vehicle speed, stability and effects traffic safety and driver comfort.

• In many countries standards, there are the minimum f value for new and maintained road.

• The minimum permissible f ranges from 0.6 in Belgium to 0.4 in France.

• Construction standards in the former SU f is determined by the expected traffic conditions and type of use. In dangerous sections: small-radius curves etc. f≥ 0.6; good conditions: f ≥ 0.45, ordinary conditions, f ≥ 0.3.

• In China, pavement skid resistance conditions research began in the 1980s, in 1996 SRV tested by a static pendulum skid-resistance tester was introduced.

• SRV limit an acceptable range, with no allowance for different pavement conditions.

• SRV of secondary roads is 47–50, with a SMTD of 0.4–0.6 mm.

2.SAFEty FACTORS BETWEEN VEHICLE AND PAVEMENT

• Safe operation of a vehicle traveling on any road must be satisfied for equations (1) and (2):

(1)

Where: Pis the vehicle traction force (kg)；Zω、Zψ、Zj are the air, road, and inertia resistance (kg).

• P is limited to the friction between the tire and pavement:

(2)

Where:GKis the vehicle load; Ψ is the adhesion coefficient between the tire and pavement.

• In a straight line, the braking resistance is parallel to the driving direction, longitudinal friction force FR is involved.

• The front wheels are turned at an angle to the driving direction, a transverse friction force FT occurs.

• FR and FT occur simultaneously, the integrated force should not exceed the maximum friction force F, as shown in equation(3).

F2 = FR2 + FT2 (3)

Figure 1: Friction between tire and pavement

• In Fig1, when Fmax is exceeded the vehicle will slide.

• Braking suddenly on a curve may cause the vehicle to exceed Fmax and accident.

• Over speed is the major cause of accidents on curved sections.

• The low f, lack of superelevation on the curves of one national highway in China causes six sliding per year.

Figure 2: Relationship between number of accidents and SFC.

• If the normal reaction forces of the front wheels drop to zero, the wheels of the front axle may raise off the ground, resulting the car’s overturning.

• If the normal reaction forces of the rear wheels are at zero the traction force is lost base on the adhesion condition, and if the automobile is unable recover, it may slide.

Figure 3:Accident rate versus SFC for rainless & rainy days

• Figure 5 showing SFC changes in kilometers0–48 of a certain highway in 3 years , opened in September 1993.

Figure 5: SFC as a function of time and location for a certain expressway

• Adequate pavement friction is a necessary condition for vehicles to travel safety.

• According to conditions of the vehicle running force, f can be divided into:

• longitudinal friction coefficientfR

• transverse friction coefficientfT

• To keep the balance between the actual and expected values, some concepts are introduced :

(1) maximum longitudinal friction coefficient fTmax

(2) allowable longitudinal friction coefficient fTA (allowed transit friction)

(3)allowable transverse friction coefficient fSA (allowed side friction)

(4) expected longitudinal friction coefficient fTR (required transit friction), and expected transverse friction coefficient fSR (required side friction).

(1) Quantitative safety model for the longitudinal friction coefficient

• The longitudinal friction coefficient can be calculated by (7)

(7)

Where:

fT—— the longitudinal friction coefficient

FT—— the longitudinal friction

Q —— the gravity.

(1) Quantitative safety model for the longitudinal friction coefficient

• fTA is a reliably safe and determined by the pavement friction-coefficient standard.

• fTmaxis the maximum value under special limiting conditions

• fTR is under the expected operating speed.

• In practice, there is probably a certain standard deviation ΔfT between fTA and fTR, which is calculated by(8)

(8)

(1) Quantitative safety model for the longitudinal friction coefficient

• When fTA ≤ fTR, while ΔfT ≤ 0, the pavement safety performance is “good.”

• When ΔfT > 0 but less than a certain “critical value,” the pavement safety performance is “relatively good.”

• When ΔfT > 0 and also more than a “critical value,” the pavement safety has hidden danger

(2) Quantitative safety model for the transverse friction coefficient

• The transverse friction coefficient value deviation ΔfS between fSA and fSR, can be calculated by (12)

(12)

• When ΔfS ≤ 0, the pavement safety performance is “good.”

• When ΔfS > 0 but less than a certain “critical value,” the safety performance is “relatively good.”

• When ΔfS > 0 and greater than a “critical value,” the pavement is “dangerous.”

• A certain road in China was divided into three sections according to pavement age: 16 years, 4 years, and new reconstruction completed in September 2003.

• In the three sections, the accident-prone portions and those with skidding accidents are chose as shown in Table 3.

• In tested sections, five representative points about 5–10 m apart along the left wheel path in the driving direction tested by the pendulum apparatus in table 4.

• Pavement structure type: 3-cm asphalt concrete, 10-cm bituminous penetration, 20-cm gradation of sand–gravel-doped lime soil.

• Table 4: Measured pavement friction coefficients

According to pavement design standard, fTA = 47.

Equation (8) for the quantitative model gave ΔfT values of 14.2, 13.2, 15.48, 4.72, and -10.2

Taking the ΔfT=Min(14.2,13.2,15.48)=13.2 as the critical value, we obtain FB20 ≥ 47 (ΔfS ≤ 0)

Effects of appraise

• The road system must meet the following equation:

• Safety: ΔfT ≤ 0, ΔfS ≤ 0,(ΔfT=fTA-fTR, ΔfS=fSA-fSR )

• good: 0<ΔfS≤min(ΔfSi), 0<ΔfT≤min(ΔfTi),

• Dangerous: ΔfS>min(ΔfSi), ΔfT>min(ΔfTi)

• The minimum f for penetration-type asphalt pavement allowable standards is too low and can easily contribute to skidding accidents in China.