access from abutting property, thus eliminating many of the roadside conflicts which confront the driver. Crossroads are grade separated, eliminating angle collisions occurring at intersections. Wide medians eliminate head-on collisions, and carefully planned interchanges with long speed change lanes minimize rear end and turning collisions. Research undertaken in the early 1950’s demonstrated that full control of access reduces accidents, injuries and fatalities by 50 percent or more.[1]
Control of access also has safety benefits on two-lane highways. A study on a two-lane rural highway with average daily traffic of about 8,000 vehicles showed that, if the number of at-grade intersections and roadside business driveways per mile were increased a hundredfold, the expected accident rate would increase approximately 14 times.[2]
Roadside Safety
One of the major accident types is the single vehicle run-off-the-road accident. Often the vehicle strikes an object such as a tree, rock, sign, or guardrail, resulting in injury or death to the vehicle’s occupants. Although studies designed to relieve this problem began in the 1920’s, the most significant advances have occurred in the recent past.
In December 1966 a short-range Public Roads research program was established for quick-payoff concepts and devices to substantially reduce the severity of single vehicle collisions with fixed roadside structural obstacles. The primary objective of this program was the development of devices that would protect vehicle occupants against severe injury during impact with rigid gore structures, bridge piers, sign bridge supports, and open areas between twin bridges. The studies emphasized the use of existing technology in such fields as impact absorption barriers, new and improved types of redirectional barriers, vehicle entrapment devices, cable supported structures, and improved computer simulation techniques. The impact acceptance criterion was tentatively set at a maximum deceleration limit corresponding to a 10-foot stopping distance from 60 miles per hour for passenger vehicles, a survivable situation for shoulder and lap-belted vehicle occupants.
Several types of impact attenuators, or crash cushions, were successfully developed and tested. These include the steel drum barrier (an assemblage of empty 55-gallon drums), a water cushion (an array of waterfilled vinyl cells), and clusters of sand-filled plastic drums. More than 3,000 such devices have been installed on the highways to date, with evidence that they are extremely effective in reducing severity of single vehicle collisions.
The overall concept and application of breakaway sign supports were also developed primarily during the past 10 years. Research efforts began with the inception of multi-State, pooled-fund studies of the breakaway sign and luminaire support concept. The research included full-scale field experiments and crash testing of vehicles into prototype breakaway sign supports. In addition, much work was also developed or verified through the use of computer simulation and theoretical studies. Private industry also joined in this effort and produced a number of breakaway concepts.
Motorists on I-84 in Connecticut are protected from a possible fatal injury at this median strip bridge pier by a guardrail, the approach end of which is buried in the ground as the result of research into the causes and severity of accidents.
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