The paper is essentially a companion to an earlier report dealing with left-hand exit ramps for freeways and is divided into three main sections' /a/ a study of the general operating characteristics of left- and right-hand entrance ramps on urban freeways; /b/ an analysis of traffic behavior along a 2-mi section or urban freeway containing two internal diamond interchanges; and /c/ a comparative study of the reported accident rates at a sample of right- and left-hand entrance and exit ramps on urban freeways in the chicago area. Brief descriptions are given of study locations and study techniques, together with a discussion of major results. Conclusions drawn concerning the operational efficiency, relative safety and general suitability of left-hand entrance and exit ramps for urban freeways under the type of site conditions existing in the chicago area include the following' /1/ none of the left-hand ramps studied caused any prolonged disruption of flow in the adjacent freeway lanes, /2/ flow conditions on a left-hand entrance forming part of an internal diamond interchange were significantly different from those on a right hand entrance carrying similar traffic volumes and forming part of an external diamond interchange, /3/ extremely high merging rates were observed at one left-hand entrance ramp without causing average left lane speeds upstream to fall below 45 mph. Supersaturated flow conditions resulted downstream. /4/ on level sections of heavily traveled urban freeway in the chicago area the average reported accident rate per million ramp vehicles was consistently higher at left-hand entrances and exits than at right-hand entrances and exits. No significant difference was found in the average severity of accidents at left and right-hand ramps. /5/ individual design characteristics appeared to have a more significant effect on the accident rate at a given ramp than ramp or freeway volume alone, and /6/ average accident rates for both left and right-hand facilities tended to be slightly though not consistently higher at entrance ramps than at equivalent exit ramps.
Samenvatting