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18 The last three of the traffic safety concerns identified Legal by survey respondents relate to obstructions and physical features. Codes in most states do not allow vehicles to use shoulder lanes for congestion bypass purposes and without enforce- Regular use of the shoulders probably would require in- ment abuses are inevitable. Concerns related to this are reg- creased efforts to remove debris from the shoulder area. ulatory authority and enforcement of abuses. Operations of wide vehicles on the shoulder also re- duces the horizontal clearance between moving traffic The implementation of bus use shoulders needs to include and bridge abutments, railings, sign posts, and other lat- a framework and facilities to enforce regulations. As most eral obstructions. abuses would typically include cheating by exiting and Drainage failures most often affect shoulder facilities entering motorists, rather than continuous travel by general rather than high-speed general traffic lanes. One respon- traffic on the shoulder, clear boundaries need to be defined dent felt that this would increase hydroplaning risks. for the exit and merge interchange movements. Good bus driver training, good operating protocols, and care- ful design would appear to address most of these issues. Costs In addition to the cost needed to upgrade shoulders to allow for bus use, concerns were also raised about the added cost Physical Design of maintenance necessary to keep shoulders free from debris and to maintain the shoulder pavement. Design practices and operating environments vary by juris- diction. Seven concerns were identified for physical design. Special issues that were raised for toll road facilities in- cluded the acceptance of toll paying motorists for the prefer- Shoulder width adequacy, ential use of the shoulders and also how to handle the shoulder Shoulder pavement strength, use at toll plazas. Signage needs, Lateral obstruction adjacent to shoulder, Need to narrow general traffic lanes, OPERATIONAL EXPERIENCE Modifications to drainage inlets compromise function, and Safety Conflicts with pavement edge rumble strips. A 1998 study of the Minneapolis bus shoulder use found: Shoulders generally have not been constructed to accom- modate regular use by large vehicles. Most modern shoulders 1. Travel time savings are quite variable and depend are 10 to 12 ft wide; however, many older highways have nar- greatly on weather and traffic conditions. The worse rower shoulders and less clearance to lateral obstructions. the weather and traffic, the greater the time savings. Buses are 8.5 ft wide excluding mirrors and approximately During snowy conditions, the 4-mi I-35 segment 10 ft wide with mirrors. Shoulder pavement typically is not between Lake Street and Highway 62 (the Cross-town) designed to the same thickness and strength as general traffic realized a 9-min travel time savings using the shoulder. lane pavement. Conventional traffic signage is not designed 2. Bus-only shoulder operations are inherently safe; from to support shoulder use operations. Drainage inlets along February 1996 to June 1996 there were only three shoulders are not designed for comfortable traverse by buses crashes, none of them serious. and reconstruction can compromise their ability to effectively 3. A 10-ft-wide shoulder is marginally adequate. Drivers remove water from the roadway. Electrical junction boxes are uncomfortable with the narrow lanes, especially also often need to be relocated. Lastly, some highways are de- during adverse weather conditions (and adverse signed with rumble strip warning edges between the general weather conditions are where travel time savings are travel way and the shoulder. These rumble strips would not the greatest). Snow and other debris that obstruct visi- support comfortable use of the shoulder and would need to be bility add to the discomfort level. removed. An issue that was not identified from the project sur- vey, but is also important concerns drainage cross slopes. NCHRP Report 369: Use of Shoulder and Narrow Shoulders typically have cross slopes greater than general traf- Lanes to Increase Freeway Capacity (3) provides the most fic lanes and the higher cross slopes increase the level of dis- extensive analysis of narrowing lanes to use shoulder comfort for bus passengers. These are all important design facilities for general traffic. This study did not address the considerations; however, they can generally be addressed by more controlled concept of allowing only buses to use the physical upgrades to the highway facility. The costs for these shoulder lanes. NCHRP Report 369 cautions against using upgrades vary widely, but are modest compared with most the right shoulder, particularly for segments with high highway widening and interchange reconstruction costs. truck volumes. It found that accident rates increase for