Operational Characteristics of Lane Drops

A lane drop is defined as a location on a highway where the number of lanes provided for through traffic decreases. For purposes of this study, the broad category of lane drops has been further subdivided into three specific classes: lane exits, lane splits, and lane terminations. These subdivisions are illustrated schematically in Figure 1. A lane exit refers to a location where the number of through lanes decreases at an interchange on a multilane roadway. A designation of lane split denotes a major fork of a multilane highway where the level of traffic service provided at the terminus of either prong is approximately equal. Thus, the lane split does not have the same exit connotation which is associated with a lane exit. The third category is the lane termination which occurs when a lane is simply terminated. A lane termination leaves a driver with no choice, he must merge into the other available lane(s). A lane termination also has no connection with an exiting situation. Associated with the first two categories, lane exits and lane splits, is the concept of driver decision. The driver who is confident of his destination and the proper path thereto generally presents no conflict with the flow of traffic. The problem arises largely from those drivers who are inattentive, intoxicated, uncertain of how to reach their destination, and(or) have improper driving habits. It is these individuals, as shown in Figure 2, who conflict with the traffic stream. Therefore, it is imperative that the driver be made aware of the necessity for an early decision regarding his course of travel. The driver who makes an errant decision and abides by it is not as dangerous as the one who makes a delayed decision and attempts, often too late, to correct it. Thus, the driver who perchance takes the wrong branch is likely to resort to desperation tactics and back up or undertake some other maneuver that is illegal or contrary to safety. The purpose of the study reported herein was to evaluate the operational characteristics of lane-drop situations as they are influenced by various fore-warning, decision-demanding messages. More specifically, the immediate purpose was to discover types of signs, pavement markings, and lane delineations which minimize or reduce erratic movements at existing lane drops. It was also hoped that an optimum design criteria for lane-drop situations might be determined. Several standard and untried traffic control devices were selected for experimentation. A pilot study at a geographically advantageous location containing three lane splits was conducted. The results of this pilot study are the subject of this report

Operational Characteristics of Lane Drops

Traffic behavior studies were conducted at seven lane-drop locations, representing four lane-drop classes. These studies were composed of conflict observations (consisting of both erratic movement and brakelight applications), spot-speed observations, and lane volume counts. Such a study was made before and after each different traffic control device installation in an attempt to determine which device was the most effective in minimizing conflicts at existing lane drops. A study of conflict deviations indicates that no single type of traffic control device studied was significantly effective in reducing erratic movement and brakelight rates at all seven lane-drop locations. Rather, it appears that different traffic control devices are generally most effective at each of the locations. The single-lane exit without taper constituted the lane-drop classification with the lowest conflict rates of the four different lane-drop classifications studied. The lane-termination classification had the next lowest conflict rates. Those lane drops with poorer sight geometries were observed to have higher conflict rates. No definitive relationship between traffic conflict rates and either traffic volumes or accident rates was found for the lane drops studied. Certain data limitations were discovered

Before-and-After Analysis of Safety Improvements on I 75 in Northern Kentucky

In an attempt to evaluate the effectiveness of various safety improvements on approximately five miles of I 75 in northern Kentucky, a before-and-after analysis of accidents was conducted. Safety improvements were: 1) a variable message signing system in the northbound direction; 2) a New Jersey-type median barrier wall; and 3) general improvements such as guardrail end treatments, break-away sign installations, contour grading, etc. Results indicate the safety improvements were effective in reducing the overall accident rate. Separate analysis also revealed that variable message signs were effective in bringing about a significant reduction in accident rates. There were no fatalities during either of the data collection periods. However, accident severity increased after the improvements. Results were influenced by low skid resistance, continued wear in wheel tracks, and increasing congestion throughout the study section

Operational Characteristics of Lane Drops

Traffic behavior studies were conducted at seven lane-drop locations, representing three lane-drop classes. These studies consisted of conflict observations (that is, erratic movements and brakelight applications), spot-speed observations, and lane volume counts. Such a study was made before and after each different traffic control device installation in an attempt to determine which device was the most effective in minimizing conflicts at existing lane drops. A study of conflict deviations indicates that no single type of traffic control device studied was significantly effective in reducing erratic movement and brakelight rates at all seven lane-drop locations. Rather, it appears that different traffic control devices are generally most effective at each of the locations. The single-lane exit without taper constituted the lane-drop classification with the lowest conflict rates. Those lane drops with poorer sight geometries were observed to have higher conflict rates. No definitive relationship between traffic conflict rates and either volumes or accident rates was found for the lane drops studied. Certain data limitations were discovered

The Safety Barrier Dilemma

Ran-off-the-road fatal accidents currently account for approximately 65 percent of all freeway fatalities (1). Accordingly, ever-increasing emphasis has been given to the development of effective safety barrier systems, from guardrails to earth berms to median barriers to energy absorbing barriers and mires. However, highway designers have also recognized that safety barriers are hazards in themselves, misfits in the highway environment, and that they are items to be eliminated wherever possible. In a study of fatal accidents on the Interstate Highway System, it was found that fixed object collisions have been the leading source of fatalities, accounting for 43 percent of the 1968-1969 fatal accidents (2). Ironically, guardrails were found to be the most frequent objects struck first -- accounting for 31 percent of the total. Furthermore, this same study estimates that, excluding non-interstate and secondary urban roads, 6,300 miles of guardrail were constructed on public roads in 1969. Statistics such as these illustrate the risks facing today\u27s drivers on the Interstate Highway System. Until a major modification is made that produces a significant reduction in such risks, less mobility (through travel restrictions) will be required to produce a significant reduction in fatalities per year (3)

Experimental Installations of Impact-Attenuating Devices

From a survey of the interstate system in Kentucky, 26 gore sites were found to be eligible for safety improvements. Energy absorbing barriers have been installed at five. Barriers are planned at 11 sites; seven sites have been contour graded; and three sites have been dismissed from consideration. HI-DRO Cushions and Fitch Inertial Barriers were found to be effective crash cushions. HI-DRO Cushion maintenance costs per impact were less than those for Fitch Inertial Barriers; however, initial costs of materials and installation were higher. The HI-DRO Cushion is generally more adaptable to narrow and relatively short areas than either the Fitch Inertial Barrier or the Steel Crash Cushion. Desirability of redirectional capabilities is dependent upon site geometrics, traffic volumes, and speeds. If there is no feasible alternative, installation of an impact attenuating device is advocated in terms of warrants

Experimental Installations of Impact Attenuation Devices

The expanding system of limited access, multilane highway facilities in the United States has enhanced traffic service between many varied origins and destinations. As a result, these facilities have not only reduced travel times and travel costs, but have generated additional travel by virtue of their technological splendor. The increasing dominance of multilane highways in terms of percentage of total vehicle miles travelled has produced a revolution in highway engineering philosophy. Higher design speeds, increased lane widths, lateral separation of opposing traffic streams, absence of passing restrictions, and increased traffic capacities were attributes intuitively sought. An ironic result is the emergence of new accident styles on roadways which had as their specific purpose the reduction of accidents. Dominant among these new styles was the single vehicle ran off the road type. Recent investigations have shown that this accident mode may be the largest single contributor to fatalities on limited access, multilane facilities. Contributing to the preponderance of this accident type are some seemingly minor but potentially deadly elements of the roadway. Previous studies have been concerned with bridge piers located in the median, the location of median crossovers, and the horizontal and vertical dimensions of medians themselves. Problems have been created by rigidly fixed objects such as bridge walls or massive sign standard bases located in the gore area of bifurcating roadways and exit ramps

An Operational Analysis of the I 64, I 65, I 71 Route Junction in Louisville

The Kennedy Interchange in Louisville is the most geometrically complicated of any in Kentucky; it is the junction of three interstate routes, I 64, I 65, and I 71. The many diverging, weaving, and merging movements demand a driver\u27s attention. One merging situation collapses into an impasse during peak-hour traffic. This study is responsive to certain inquiries by the Department concerning safety and possible discovery of design deficiencies at this interchange. It was not intended to be an in-depth study but rather an exploratory identification of problems and their locations; more comprehensive studies, by others, would ensue if needed. The specific objectives were to discover conflicts and erratic movements and to locate and analyze high-frequency accident sites