## 2 Literature Review
### Small Spacing Interchange Improvement Methods  
Several scholars have investigated the relationship between ramp entrance-exit spacing and freeway accident rates, and the results have shown that the accident rate increases as the ramp spacing decreases [28; 29]. It has also been suggested that entrance ramp density has a greater impact on freeway accidents, whereas the effect of exit ramp density is not statistically significant [30]. Current research on interchange spacing is dominated by ramp geometric design and ramp spacing studies. Some scholars have given ramp spacing recommendations for two different ramp combinations: \"entrance-exit\" and \"exit-exit\" [31]. Through theoretical derivation and simulation, [PERSON] proposed the spacing between tunnel exits and interchange exits according to varying traffic volumes and diverging rates and comprehensively evaluated the traffic efficiency and pollutant emissions of different schemes based on entropy [32]. Based on a data envelopment analysis combined with a hierarchical analysis, the optimal spacing is proposed for the four ramp combination types of exit and entrance [33]. A scholar has comprehensively considered multiple aspects, such as safety, accessibility, and cost, and developed a dual-objective optimization model for optimizing the layout of highway interchanges [34]. [PERSON] proposed the entropy-weighted TOPSIS evaluation method to study the reasonable spacing between interchanges from four aspects: traffic efficiency, safety, accessibility, and economy [35]. [PERSON] conducted an analysis of the impact of entrance and exit ramps on the mainline from a mathematical perspective based on the theory of linear stability. They proposed a traffic grid model to study this phenomenon [36]. In order to improve traffic safety in sections with dense spacing between entrance and exit ramps, many scholars have focused on controlling the spacing between ramps based on traffic volume. However, [PERSON] took a different approach by focusing on improving geometric design. They studied the impact of lane configuration, number of through lanes, spacing between ramps, shoulder width, and other factors on enhancing traffic safety [37]. [PERSON] conducted research on the variation pattern of lane-changing distance with changes in traffic volume and diverging ratio. The author found that for every 100 pcu/h increase in traffic volume per lane, the lane-changing distance should be increased by 50-100 m. The research findings provide valuable insights for the design of interchange spacing [38]. In order to improve the operating conditions in the weaving area, lane utilization and traffic efficiency can be effectively improved by installing continuous auxiliary lanes between the ramps [39] and reducing the accident rate. However, the safety benefits of installing auxiliary lanes tend to decrease as the ramp spacing increases [28; 40]. For compound interchange, the safety of type A, B, and C weaving areas is evaluated by building a collision prediction model. The prediction results show that the C-type weaving area exhibits the highest level of safety, and the B-type weaving area has the lowest [2; 18]. In addition, it has been found that although a decrease in ramp spacing leads to an increase in collision frequency, drivers will appropriately reduce their speeds in order to avoid an insufficient lane change distance. This adaptive behavior by drivers helps mitigate the severity of collisions [40]. [PERSON] concluded that operating conditions could be effectively improved by extending the auxiliary lanes between the entrance and exit ramps outside the weaving area [41]. Some scholars have also proposed methods of managing interchange hazardous areas using signage and markings. The downstream sections of urban interchanges with short weaving areas were found to be high-risk. The traffic conditions in the high-risk sections were optimized by shifting the traffic risk to areas with better operating conditions.