Project Objective

The objectives of this research are to assess the effect of the increased speed limit on safety, travel speeds, and other unanticipated impacts including the shifts in lane distribution, traffic diversions, and spillover effects - such as, increase in average speeds and crashes in highway sections that did not increase their speed limits.

Project Abstract

One of the main objectives of the proposed research effort is to conduct a comprehensive and critical review of the literature to assess the impacts of increased speed limits. There has been extensive research on the impact of speed limits on safety and average speeds, although some of these studies have been criticized for

(i) unavailability of disaggregate data, especially on speeds,
(ii) lack of control for confounding factors, and
(iii) inappropriate use of statistical methods (see McCarthy, 1998).

Hence, the proposed study will contribute in at least two areas

(i) critically evaluate the literature and identify studies that have utilized a valid methodology by using appropriate data, controlling for confounding factors, and used appropriate statistical methods, and
(ii) conduct a review of the literature to identify any unanticipated impacts of speed limit changes, such as, changes in lane distribution, diversion of traffic to high speed routes, and spillover effects.

The activities will also include a review of the literature on microscopic traffic flow models to understand driver behavior under different traffic conditions.

Task Descriptions

Task 1. Literature Search. This task will focus on a comprehensive and critical review of studies that have looked at the unanticipated impacts of changes in the speed limit. These unanticipated impacts could include: (i) diversion of traffic between slower to higher speed roads, (ii) spillover effects - changes in average speeds and crashes in roads that did not alter their speed limits, and (iii) change in the lane distribution. The literature review will specifically try to answer the following questions

(i) Does the traffic flow more uniformly when the speed limit is increased ? in other words, does speed dispersion reduce with increase in the speed limit? (ii) Has the traffic shifted in lane distribution to the right ? in other words, has the 65 mph speed limit resulted in fewer vehicles traveling in the left lane? (iii) What are the impacts of speed changes in areas where there are limited acceleration and deceleration lanes (speed-change lanes)? Does the higher percentage of traffic in the right lane make it more difficult for traffic to enter and exit the highway?

Task 2. Develop Request for Proposal (RFP). This task will develop an RFP if requested by the RSIP. The objective of the RFP would be develop a project statement to collect and analyze field data in New Jersey to study the effect of speed limit on lane distribution and other unanticipated impacts. The RFP will be developed in close coordination with the RSIP in the specific format required by NJDOT.

Task 3. Prepare reports. Quarterly progress reports will be prepared according to the format specified by NJDOT. Reports will also be prepared to summarize the literature review. A draft final report will be prepared for review by the RSIP. After receiving comments from the RSIP, the final report will be prepared incorporating the reviewer?s comments and delivered in hard copy, MS word 97, adobe pdf format, and on CDROM

Student Involvement

Both graduate and undergraduate students will participate in the different activities. Their involvement would include data collection, data analysis, and computer applications.

Other Research Activities

None

Technology Transfer Activities

This is a technology transfer effort by doing a literature review. If requested by the RSIP, an appropriate implementation and training plan will be included as part of the RFP.

Benefits of the Project

This project will produce two main products. The first product will be a technical memorandum based on the review of studies that have looked at the effect of speed limits on average speeds, speed dispersion, traffic safety, lane distribution, traffic diversion, and spillover. This document can be used by researchers and state DOT?s to understand the possible implications of changes in the speed limit.

The second product will be the development of an RFP that can be used to facilitate the collection of field data to assess the effect of speed limits on lane distribution.These products along with results from the analysis of the field data can be used by New Jersey to determine if it will be appropriate to continue the 65 mph speed limit.