Click for next page ( 58


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 57
Treatment Descriptions 57 4.11.5 Safety Effects The documented safety effects of reduced lane widths generally address segment applications, although some intersection approach data may be included. Despite inconsistent data, concern remains that narrower lanes may be accompanied by reduced safety. The two principal aspects to the potential link between lane width and safety are as follows: Wider lanes increase the average separation between vehicles moving into adjacent lanes and may provide a wider buffer to accommodate small, random deviations from drivers' intended paths; and Wider lanes may provide more room for correction maneuvers by drivers in near-crash cir- cumstances. (Hauer, 2000) Research and analysis conducted for NCHRP Project 3-72 found no decrease in safety at mid- block locations or intersection approaches with narrow lanes except in the following limited cases: Four-lane undivided arterials with lane widths less than 10 ft, Four-lane divided arterials with lane widths less than 9 ft, and Four-leg stop-controlled intersections with lane widths less than 10 ft. Research by Harwood et al. (2000) developed accident modification factors (AMFs) for crash types related to cross-section elements (including opposite direction, and single-vehicle run-off- road) for two-lane highways. The AMFs range from 1.05 to 1.5 for 9- to 11-ft lanes compared to 12-ft lanes, indicating that two-lane facilities with narrower lanes would be expected to experi- ence higher crash rates for certain crash types. (Harwood et al., 2000) The AMF of 1.50 for 9-ft lanes implies that a roadway with 9-ft lanes would be expected to experience 1.5 times as many crashes as a roadway with 12-ft lanes. However, some studies indicate that lanes wider than 12 ft may also be associated with higher crash frequencies. 4.12 Visible Shoulder Treatments 4.12.1 Overview No test sites provided documented applications for the high-speed intersection treatments discussed in this section. Visible shoulder treatments heighten driver attention by narrowing the visual width of the roadway. These treatments can be applied at transition areas, road- ways with existing shoulders, and roadways in rural areas with no sight line limitations. There are many design variations. Secondary effects and considerations should recognize that bicy- clists using the shoulder may be impacted by these treatments, depending on the materials used. 4.12.2 Applicability and Considerations Visible shoulder treatments are used to change the appearance of the paved area to either aes- thetically blend roadway facilities into the surrounding landscape or to create a contrast between the shoulder and travel lane to heighten drivers' awareness of the roadway. Visible shoulder treatments can consist of shoulder pavement markings, pavement coloring, shoulder composition, and shoulder rumble strips. If introduced in advance of the intersection, these treatments may be used to alert drivers of the change from roadway segment to the intersection influence area. Exhibit 4-25 shows an example of a visible shoulder treatment using brick.