Performance Assessment of Road Barriers in Indiana

Road barriers have been used as an effective countermeasure to prevent exposure of errant vehicles to both vehicles travelling in the opposite direction and to roadside hazards. The objective of this study was to evaluate the in-service safety performance of three types of road barriers (concrete barriers, steel W-beam guardrails, and high-tension cable barriers) in Indiana using cross-sectional analysis based on crash data. The quantitative evaluation was comprised of three components: 1) the effect of the road, barrier scenarios, and traffic on the barrier-relevant (BR) crash frequency, 2) the effect of the road and the barrier scenarios on the BR harmful events, and 3) the effect of the BR events and other conditions on the injury outcomes. The introduction of the BR harmful events linked the crash onset with its outcome. The three developed statistical models were connected through their inputs-outputs and followed the sequence of various BR events during the BR crash. This improvement allowed a more comprehensive and insightful analysis of the barriers’ safety effects and a more efficient use of data. The injury outcomes were estimated for all the individuals in a crash rather than for the most severe outcome of a crash. Further improvement of the cost estimates was accomplished by utilizing hospital data. For median barriers, this study found that the total number of BR crashes was higher with the use of median barriers, mostly due to the introduction of collisions with barriers and an increase in the collisions after redirecting vehicles back to traffic. These undesirable effects of barriers were surpassed by the positive results of reducing hazardous events such as cross-median crashes, rollover events, and collisions with roadside hazards, which substantially reduced the number of severe injuries and fatalities. The average (unit) crash costs were estimated for roads without barriers and for roads with various barrier scenarios. The crash costs were reduced by 50% where cable barriers were in medians wider than 50 feet and where concrete barriers or guardrails were in medians less than or equal to 50 feet wide. Roadside barriers (guardrails) reduced the unit crash costs by 20% to 30%. Median cable barriers were found to be the most effective among all the studied barriers due to the smallest increase in the crash frequency and least severe injuries in barrier-relevant crashes. A cable barrier’s offset to the travelled way was also investigated in this study. When considering vehicles moving in one direction, the nearside cable barriers installed at an offset less than or equal to 30 feet performed better than far-side cable barriers with a larger offsets thanks to the better protection they provide for vehicles against rollovers in the median and impact with the median drain. Consequently, the biggest safety benefit can be expected where cables barriers are installed in the median at both edges. The results were implemented through a set of crash modification factors and unit crash costs estimated for 51 road-barrier scenarios. An implementation procedure is provided to quantify the crash costs and the safety benefits for these scenarios

SNIP Light User Manual

A systemic approach to identifying road locations that exhibit safety problems was provided by the Safety Needs Identification Program (SNIP and SNIP2) developed by the Purdue University Center for Road Safety (CRS). The new version SNIP Light has been developed to provide other uses with planning level traffic safety analysis capability for a wider range of uses including Metropolitan Planning Agencies (MPOs) who want the tool for planning cost-effective safety programs in their metropolitan areas. The SNIP Light reduces the demand of computing and data storage resources and replaces the SQL server database system an integrated module coded in-house which is considerably faster than the original component. Furthermore, certain proficiency required to install and use the old version is no longer needed thanks to the intuitive single-window interface and executing file operations in the background without the user’s involvement. Some operations, such as optimizing funding of safety projects, are removed to simplify the tool

Safety and Operational Impacts of Alternative Intersections

As the degradation of service at some conventional intersections increases, there becomes a need for alternative solutions other than expensive interchanges. Many alternative intersections have been proposed in the past. Under certain traffic and local conditions some solutions are more promising than other. In some cases, the conventional intersection may still be the optimal choice. The presented research focused on developing guidelines that would help planners and designers identify the most promising solutions for further analysis. This objective has been addresses in two ways. Firstly, the existing knowledge on alternative intersections has been identified. Secondly, the performance of conventional and alternative intersections under a range of Indiana traffic conditions has been evaluated using micro-simulation model - VISSIM. Although a large number of sources could be found on the research subject, the existing knowledge about performance of alternative intersection design is incomplete. Only a few designs proposed in the past have been applied at a considerable number of locations including roundabouts, median U-turns, and jag-handle intersections. Other types still await implementation. The available sources are not comprehensive and deal with conditions that might be different from Indiana. The knowledge of the safety impact of these intersections is very limited. A large number of more than 1,300 scenarios were simulated runs performed with VISSIM calibrated to Indiana conditions. The simulated types of intersections included: conventional, roundabouts, jag-handle near-sided and far-sided, median U-turns, and continuous-flow intersection. Except roundabouts, all other intersections were signalized to test their capacity limits and delay-based performance. Although the roundabouts were the lowest delays at low volumes they also reached the capacity before other did. The most promising solutions for heavy volumes are median U-turns and continuous-flow intersections. The presented research developed guidelines for using alternative intersection designs. The guidelines compile the existing knowledge found in existing publications and research reports with the simulation experiments performed with VISSIM. The guidelines are ready to use and will help planners and designers determine which intersection types are the most promising under considered conditions and should be considered in a detailed way. The simulation results have been summarized in an easy to use format of graphs

Performance of Alternative Diamond Interchange Forms: Volume 1—Research Report

Service interchanges connect freeways to arterial roads and are the backbone of the U.S. road network. Improving the operations of service interchanges is possible by applying one of several new solutions: diverging diamond, single point interchanges, and double or single roundabout diamonds. VISSIM was used to perform 13,500 experiments to simulate the traffic performance of the studied alternative interchanges during a typical day for a wide range of geometry and traffic scenarios. Five performance measures were investigated: daily-average delay, level of service of critical movement, daily-average number of stops, longest off-ramp queue, and longest crossing road queue. The obtained daily-average delays at the alternative interchanges were consistent with expectations. Roundabouts had the highest average delay while single-point interchanges had the lowest average delays. Roundabouts exhibited the lowest numbers of stops among all the alternatives in the low traffic range up to non-freeway 30,000 veh/day. Diverging diamonds tended to have the shortest and roundabouts tended to have the longest queues on their off-ramps. Overall, single-point interchanges had the shortest queues among all the alternatives. The study developed guidelines for early stage screening of alternative diamond. The guidelines exhibit performance measures for 25 traffic and geometric scenarios and a wide range of traffic volumes. The guidelines provide a fair comparison procedure for alternative diamond interchanges in the preliminary planning and conceptual design stages

Performance Measure That Indicates Geometry Sufficiency of State Highways: Volume I—Project Scoring and Network Screening

The Indiana Department of Transportation (INDOT) selects projects for implementation by taking into account several criteria related to cross-section, alignment and safety to evaluate various geometry improvements to be carried out as a part of projects. The existing practice uses an intuitive point allocation method to score these individual categories. The current study proposes a methodology to evaluate these projects considering the safety and mobility impacts of the improvements which lie in the scope of each project. This methodology is also used to screen roads based on existing geometry deficiencies with respect to a desirable design standard. The road screening process and the project evaluation process form two steps that support the asset management process. The road screening process helps in filtering road segments based on geometry deficiencies and identifies the least adequate road segments. Projects may be further developed with estimated improvements to be carried out on such segments using detailed information regarding these improvements. As part of the study, we have also investigated feasible ways of extracting additional elements from available datasets to support asset management in Indiana. We have looked into available data sources for their suitability. We have combined the use of orthophotos, LiDAR point clouds, digital elevation and surface models to identify remote sensing methods that are capable of extracting the required features efficiently. We propose a framework for determining the paved surface, average grade, embankment slopes, extracting the obstructions near the traveled way like trees and man-made structures

Performance of Alternative Diamond Interchange Forms: Volume 2—Guidelines for Selecting Alternative Diamond Interchanges

Service interchanges connect freeways to arterial roads and are the backbone of the U.S. road network. Improving the operations of service interchanges is possible by applying one of several new solutions: diverging diamond, single point interchanges, and double or single roundabout diamonds. VISSIM was used to perform 13,500 experiments to simulate the traffic performance of the studied alternative interchanges during a typical day for a wide range of geometry and traffic scenarios. Five performance measures were investigated: daily-average delay, level of service of critical movement, daily-average number of stops, longest off-ramp queue, and longest crossing road queue. The obtained daily-average delays at the alternative interchanges were consistent with expectations. Roundabouts had the highest average delay while single-point interchanges had the lowest average delays. Roundabouts exhibited the lowest numbers of stops among all the alternatives in the low traffic range up to non-freeway 30,000 veh/day. Diverging diamonds tended to have the shortest and roundabouts tended to have the longest queues on their off-ramps. Overall, single-point interchanges had the shortest queues among all the alternatives. The study developed guidelines for early stage screening of alternative diamond. The guidelines exhibit performance measures for 25 traffic and geometric scenarios and a wide range of traffic volumes. The guidelines provide a fair comparison procedure for alternative diamond interchanges in the preliminary planning and conceptual design stages

Predicting the Impact of Changing Speed Limits on Traffic Safety and Mobility on Indiana Freeways

After repeal of the National Maximum Speed Limit Law, states were allowed to set individual speed limits on their interstate roads. Several states opted for a uniform speed limit while others implemented differential speed limits. The current speed limit on Indiana rural freeways limits speed of passenger cars to 70 mph and restricts to 65 mph speed of vehicles with a gross weight of 26,000 pounds or more. Indiana’s speed limit on urban freeways is mostly 55 mph, but varies from 50 mph on certain downtown sections to 65 mph on some suburban sections. Previous studies comparing uniform and differential speed limit settings as to safety and mobility produced inconclusive or conflicting results. This study evaluates the safety and mobility effects of alternative speed limit scenarios on Indiana interstate freeways. Differences in travel time, vehicle operation, and traffic safety were used to compare the speed-limit scenarios. The effect of speed limit was evaluated in hourly periods. The traffic conditions in these periods were classified as uncongested, intermediate, and congested and the speed limit effects were analyzed in relation to these conditions. Rural and urban freeways were analyzed separately and distinct speed models were developed for cars and trucks. Safety was estimated by probability of crash and the conditional probability of crash injury severity. Speed limit was found to affect mobility and safety mostly in non-congested traffic conditions, while no significant effects were found in congested conditions. A limited effect was detected in intermediate traffic conditions on rural freeways. Results indicate that replacing the differential 70/65 mph speed limit on Indiana rural roads with the uniform speed limit of 70 mph may be beneficial for both safety and mobility. Increasing speed limits on urban interstates is confirmed to be beneficial for mobility but detrimental to safety

Using Emerging and Extraordinary Data Sources to Improve Traffic Safety

The current safety management program in Indiana uses a method based on aggregate crash data for conditions averaged over several-year periods with consideration of only major roadway features. This approach does not analyze the risk of crashes potentially affected by time-dependent conditions such as traffic control, operations, weather and their interaction with road geometry. With the rapid development of data collection techniques, time-dependent data have emerged, some of which have become available for safety management. This project investigated the feasibility of using emerging and existing data sources to supplement the current safety management practices in Indiana and performed a comprehensive evaluation of the quality of the new data sources and their relevance to traffic safety analysis. In two case studies, time-dependent data were acquired and integrated to estimate their effects on the hourly probability of crash and its severity on two selected types of roads: (1) rural freeways and (2) signalized intersections. The results indicate a considerable connection between hourly traffic volume, average speeds, and weather conditions on the hourly probability of crash and its severity. Although some roadway geometric features were found to affect safety, the lack of turning volume data at intersections led to some counterintuitive results. Improvements have been identified to be implemented in the next phase of the project to eliminate these undesirable results

A Systematic Approach to Identifying Traffic Safety Needs and Intervention Programs for Indiana: Volume II—SNIP2 User Manual

This report presents the results of JTRP Project: “A Systematic Approach of Identifying Safety Intervention Programs for Indiana (SNIP2),” which aimed to develop SNIP2 to support identification of roads that have excessive crashes of the types defined by the user. In addition, this tool is capable of selecting the best combination of high-crash roads and relevant safety interventions that maximizes the safety benefits and keeps the total cost within the budget and other user-defined constraints. Unlike other studies considering the implementation time of safety projects, the optimization objective of SNIP2 is to identify an optimal combination of countermeasures renewable within a long time horizon. This simplification is accomplished by representing the projects through their annualized costs and benefits. It allows consideration of many projects for large road networks and it makes the SNIP2 suitable for identification of safety focus areas in strategic safety plans. The SNIP optimizer – a heuristic approximation of a large-size mixed integer knapsack problem based on a greedy search was extensively tested and evaluated. It was found producing optimal or near-optimal solutions in a sufficiently short time. Another research result is a comprehensive catalog of countermeasures for Indiana – a list of countermeasure names, road and crash conditions for the countermeasure relevance, corresponding crash modification factors, and countermeasure costs. The SNIP2 is computer software developed with close collaboration with the INDOT future users. It includes an updated crash and state road database. A user’s manual describes on the necessary details of the software and various aspects of its use. Two example studies are also included in the manual to illustrate its use and to better presents the SNIP2 features