# Estimation of Optimal Speed Limits for Urban Roads Using Traffic Information Big Data
## 4 Estimation of Traffic Accident Reduction Effect and Application Method  
### Speed Limit Test and Comparison  
The purpose of the test and comparison approach is to examine the possibility that the proposed model equation is suitable for application to road sections with a high accident rate. In the field, the speed limit is estimated in units of 10 \(\mathrm{km/h}\) and may not lead to an actual decrease in the operating speed limit. However, there are differences in the average speed between the low (45 sections) and high (27 sections) accident rate groups. Figure 18 shows the accident rate distribution obtained by analyzing the difference between the operating speed and recommended speed limit for road sections with low, middle, and high accident rates. For the high accident rate sections, the speed limit was generally estimated to be low (large speed difference).  
As shown in Table 13, the test results show that the speed difference between the high and low accident rate groups follows a normal distribution, leading to the conclusion that there is no significant difference between these groups at a significance level of 0.05. However, since the significance level was 0.086 (i.e., an 8.6% probability of being equal), it can be concluded that they are different at a significance level of 0.1. This translates to the proposed speed limit estimation being lower than the current operating speed for high accident rate sections. The average estimated speed difference (PSL-RSL) across the  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**In(Variable** & **DF** & **Regression** & **Standard** & **t-Value** & **Significance** & **Variance Inflation** \\
**Name)** & **Def** & **Coefficient** & **Error** & **Probability (Pr \textgreater{} \textbackslash{} | 1|)** & **Factor (VIF)** \\ \hline constant value & 1 & 0.000 & 0.000 & \(-\)1.010 & 0.321 & 0.000 \\ FC & 1 & 0.995 & 0.003 & 286.140 & \textless{}0.0001 & 0.249 \\ CD & 1 & 1.000 & 0.002 & 610.040 & \textless{}0.0001 &  
Although accident severity is not reflected, it is evident that the frequency of accidents decreases in response to speed reductions (Figure 20).  
The anticipated relative change in accident frequency as a result of changing the relative speed is expressed as:  
\[\text{y}=0.8851\times\text{X}^{1.2228}, \tag{12}\]  
where y is the relative change in the accident rate and X is the relative change in speed.  
When the speed limit (in 10 km units) determined by the proposed model equation is compared against the current speed limits, an average reduction of 11.1 km/h is recommended. It is expected that the mean travel speed for the analyzed road sections (47.8 km/h) would decrease by 25% with this recommended speed limit reduction (2.8 km/h; reduction rate: 0.94). The accident reduction effect corresponding to this estimated reduction in mean driving speed was calculated (using Equation (12)) as 18%.  
Considering the results of the study in the context of accident severity [36; 37], it is estimated that deaths would decrease by 17-24%, serious injuries by 11-17%, minor injuries by 6-9%, and property damage accidents by 5-6%.