This research project examined the speed characteristics of passenger cars and vehicles with high centers of gravity on freeway connectors to determine any discernable differences between the two vehicle types. Data to determine compliance with posted advisory speed limits and average speeds at points along connector ramps were collected on freeway connectors in Houston. Four different vehicles were driven through the curves at varying speeds while monitoring a manual ball-bank indicator as well as collecting lateral acceleration data electronically. The project determined that the general public often exceeds the posted advisory speed limits, often by more than 10 mph. While there are no seemingly discernable differences in lateral accelerations by different types of vehicles for a given speed along a curve, there may be a 5 to 10 mph difference in the driver's comfortable speed between vehicle types. The findings of this project indicate that there may be differences between the maximum comfortable speeds that drivers of heavy vehicles and passenger car type vehicles will accept for a freeway-to-freeway curve. The following conclusions confirmed by this project are applicable to freeway-to-freeway connectors and should be considered in their design, and especially in their re-design: to provide adequate deceleration and acceleration distances for tractor-trailers and other heavy vehicles, to reduce the side friction demand on trucks in the curve by developing superelevation more on the tangent, to place curve advisory speed signing with more regard to the deceleration needs of trucks. The results of this project indicate that modifying the current advisory speed setting criteria to use a 10-degree level to set a truck advisory speed and a 13-degree level for setting a more realistic passenger car advisory speed may be appropriate to more closely represent the 85th percentile speed of each vehicle on a curve.
Abstract