Safety strategies for expressway intersections
Current safety approaches for four-lane divided highways (expressways) often deal with recognition, such as flashers and signage. If those approaches don't work, the next step often is to install a signal, said Professor Tom Maze of the Center for Transportation Research and Education (CTRE) at Iowa State University. But an Iowa DOT before-and-after study found that the number of crashes stayed the same after signal installation, and the number of fatalities actually rose. Are signals the right answer?
To help answer this question, Maze analyzed 644 such expressway intersections and conducted a nationwide survey of other state agencies to understand current design and operational practices. Providing all the benefits of a high-speed interstate without the cost of expensive interchanges at all points of access, these expressways are the fastest-growing segment of the U.S. highway system, Maze said. Access along these expressways typically consists of a full interchange only at junctions with another expressway or interstate highway, with at-grade access at all points in-between. The majority of the at-grade intersections are typically controlled by a two-way, stop-control.
A Mn/DOT study cited in CTRE's report found that left-turn median acceleration lanes significantly reduce the rear-end crash rate.
Given the increasing amount of expressway mileage, Maze said, it is important to understand the safety performance of the two-way stop-control intersections and identify proven strategies to improve safety. He found that many strategies are being used. "A lot [of states] are doing the conventional things, some states are trying policy actions, and some have educational efforts," he said. (For information about University of Minnesota research in this field, see related article.)
His findings led to the creation of a table by CTRE that gives 17 different safety strategies for expressway intersections, ranging from low-cost solutions like signing and marking, all the way up to the need for grade separation. The spreadsheet includes a description of the strategy, requirements, what it applies to, and the safety benefits. The countermeasures represent a continuum of strategies that can be applied at existing intersections or can be incorporated into the planning and design process for future locations, he said.
There are options other than the conventional fixes, Maze concluded. He also recommended creating implementation policies, such as lifecycle strategies for an intersection. Download Rural Expressway Intersection Synthesis of Practice and Crash Analysis (3 MB PDF).
Responding to a question about roundabouts, Maze said there isn't enough data for a firm judgment, but thinks they "are a great safety improvement, and somebody ought to try [them]."