The relationship between side friction factor and longitudinal grade is investigated for horizontal curves considered in three dimensions, and new models for estimating side friction factors for different vehicle types are presented. The study consists of three parts: (1) a series of simulation tests using the CarSim and TruckSim multi-body simulation software packages; (2) multiplex regression analysis; and (3) presentation of recommended formulas for the side friction factor. Two different driving behaviors are considered in the simulation process: in one, the driver negotiates the curve at constant speed; in the other, the driver brakes while passing downgrades. Side friction factors are also studied separately for each axle of a vehicle. Greater side friction factors are found on downgrades for all vehicle types when cornering. From the viewpoint of skidding, the most critical situation is found for the rear axle of a sedan. In addition, braking is found to have a significant effect on side friction factors. The results and models from this study can be used in the geometric design phase by highway engineers to design safer roads.

The diverging diamond interchange (DDI) is in widespread use all around the United States and is the most popular alternative design for failing conventional interchanges. While the DDI does well in efficiently moving large left-turn demands, it is not the best design for through traffic or pedestrians. This paper proposes the new super DDI as a substitute for service interchanges which can mitigate the limitations of the typical DDIs. Our study of the new super DDI included a comprehensive series of simulation tests in VISSIM to consider different aspects of the traffic operation. We also estimated the costs of constructing a super DDI and compared those costs to other interchange designs. The results confirmed that the proposed super DDI could perform significantly better than the existing designs under the conditions tested. Overall, the super DDI showed an 18% lower travel time and a 49% higher rate of completed tests (as an indicator of the capacity) when compared with a typical DDI. However, the construction cost was estimated to be higher. Hopefully, the results show that the proposed super DDI has potential as a promising alternative design and will inspire further research and testing.