## 2 Establishment of Evaluation Index System
### Construction of Evaluation Index System  
The selection of evaluation indicators should be based on the analysis of traffic safety risk sources on long-steep downgrade sections, including factors such as people, vehicles, roads, environment, management, etc. However, considering the limitations and hysteresis  
Figure 1: Establishment process of traffic safety risk evaluation index systems for long-steep downgrade sections. of people's perception of traffic safety risks, risk evaluations based only on risk factors are not comprehensive. Therefore, this study divides the system risk evaluation index of long-steep downgrade sections into two parts--accident indicators and hidden danger indicators--so as to ensure a more comprehensive reflection of the safety risk status of the system. 1. Accident Indicators  
Accident indicators are mainly formulated according to the frequency, nature and consequences of traffic safety accidents on long-steep downgrade sections. The indicators used to characterize the degree of accident risk include the number of accidents, the number of fatalities, the number of injuries, economic losses, etc. This study comprehensively considers the influence of the length, the traffic volume and the evaluation period of the long-steep downgrade sections, and selects the accident rate per 100 million vehicle kilometers as the accident index for the traffic safety risk evaluation of the long-steep downgrade sections. 2. Hidden Danger Indicators  
Hidden danger indicators are a measure of system safety risk factors from the perspective of the system as a whole. It is a thought about safety in advance. It is necessary to eliminate traffic accidents as much as possible. The safety risk is evaluated mainly from people, vehicles, roads, environment, management and so on. The hidden danger indicators that reflect the traffic safety risks of long-steep downgrade sections can be selected by analyzing the traffic safety risk sources of the long-steep downgrade sections. The indicators are screened, eliminated and combined with the expert investigation method, and the index system of traffic safety risk evaluation for the long-steep downgrade sections is obtained as shown in Figure 2.  
Due to the different nature of traffic safety risk evaluation indicators on long-steep downgrade sections, there are both quantitative and qualitative indicators. In the evaluation, quantitative indicators can give specific mathematical calculation formulas, while qualitative indicators that are difficult to quantify only give the specific content of qualitative analysis. The following describes the individual evaluation indicators one by one. * Accident indicator \(A\)  
Traffic accidents in long-steep downgrade sections of operating expressways can directly reflect their traffic safety conditions, so the 100 million vehicle kilometers accident rate is selected as the \(A_{1}\) indicator value. This indicator can be calculated by the following Formula (1):  
\[R_{N}=\frac{N}{365\times V\times L}\times 10^{8} \tag{1}\]  
Among them:  
Figure 2: Traffic safety risk evaluation index system for the long-steep downgrade sections. \(R_{N}\)--Indicates the accident rate of 100 million vehicle kilometers (number/100 million vehicle kilometers);  
\(N\)--Indicates the number of accidents per year in the road section (number/year);  
\(V\)--Indicates the traffic volume of the road section (veh/d);  
\(L\)--Indicates the length of the road section (km). * Hidden danger indicators
* Unsafe road conditions \(B\)
* Alignment conditions of the road \(B_{1}\)  
I. Average longitudinal slope and continuous slope length \(B_{11}\)  
This indicator is aimed at the average longitudinal slope and continuous slope length of any section of long-steep downgrade sections, and it is evaluated according to the definition standards and specification requirements of long-steep downgrade sections of expressways. II.