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5 CURBING AND CLEARANCES 5.3 Ranier Curbs Barrier curbs are relatively high, are steep (vertical) facer], and are de- signed to discourage vehicles from leaving the roadway. In the usual 6 to 8 inch (15 to 20 cm) high range they serve as a deterrent to vehicles crossing over them but cannot pre- vent such a move.22 Furthermore. curbs over 8 inches high may restrict Opening of automobile doors for pickup or discharge of passengers. It crossing is to be physically pre- vented, a barrier height of at least 32 inches (BO cm) is required to restrain most passenger vehicles .31.25 These high barriers (concrete or steel plate) should be of the "New Jersey" type.3%.1Z1 See Figure 5.2. The value of replacing a low center island with a New Jersey barrier on roadways with vehicle running speeds up to 50 MPH (80 KPH) is shown in the tabulation of fatalities resulting from head•on collisions for the before- and -after Condition in New Jersey.32 Location ADT Before Atter Route 4. 6 in Englewood 50.000 3'h yrs. Route 22, 11 in Hillside 59.000 3 yrs. 1 in 6 yrs. 0 in 7 yrs. Although a New Jersey barrier mate- rially reduces the number of head -on collisions. there may be a negative tradeoff where other accidents such as same - direction types increase after the installation.13 A low, 1 to 2 foot (0.3 to 0.6 m) wide , barrier curb should not be used as a walkway on bridges. If a sidewalk is needed. it should be similar to the design used along the balance of the street. In tunnels and along the faces of high walls, a walkway with barrier curb. having a minimum width of 18 or 24 inches (0.5 to 0.6 m) desirably. should be provided for emergency use by pedestrians. 5.4 Mountable Curbs Mountable curbs are used where it is preferable to permit moving vehi- cles to cross them at locations such as pavement edges adjacent to a grassed berm or median, at the nose Figure 5.2. New Jersey Barrier. of a gore, at the edges of chan- nelized islands, etc. They are usually 2 to 6 inches (5 to 15 cm) in height, with rounded or plane sloping faces. The lowest 1 inch (2 cm) can be a vertical face to allow for future pave- ment resurfacing. They may be con- structed as extruded curbs. Flush medians (i.e.. no curbs) should be considered a viable alternative to mountable curbs in medians where it is intended to provide for turning movements across the median. A mountable curb should not generally be used where it is intended to utilize the median as a turn lane. Mountable curb should be designed so that an unsuspecting motorist who hits one will not lose control of the vehicle or damage the undercar. nage. The same applies to a dished gutter adjacent to or as part of a mountable curb. A well - publicized multiple fatality accident is thought to be directly related to a low, un- needed barrier curb 21 5.5 Visibility The ability of motorists to see curbs at night is important. Everything else being equal. a vertical curb is gener- ally more readily visible than a mountable Curb, chiefly because of its greater height. In order to improve Curb visibility, as much contrast as possiole should be provided between the curb and the adjacent pavement, shoulder, or berm. Methods of accomplishing this include: Page 11 1) Use of white cement and light- colored aggregates on the outer surfaces of concrete curbs 2) Reflector faces (depressions and ribs) formed On the outer surfaces or concrete curbs 3) Application of reflec:orized ma. terials on asphalt and concrete curbs 4) Insertion of reflector buttons on the top of the curb. etc. Occasional cleaning or repainting of the exposed surfaces of the curbing is necessary to maintain adequate visibility because of splash. dirt. and grime. In areas where there is little or no snowfall, it may be preferable to use a pavement edge line instead of re- flectorizing the curb. 5.6 Curbs at Driveways In many instances, driveway open- ings are created by lowering the top of a vertical curb to within 1 or 2 inches (2 to 5 cm) of the gutter level, Dropping of the curb at each end of the opening is accomplished in a length of 12 to 18 inches (0.3 to 0.5 m). With this type Of construction there may oe no radius between the street curb and the driveway curb or apron. In addition. the driveway opening is less visible to the motorist than it would be with a radius. Such a curb treatment is not recommended for high- volume residential. com• merciat. or industrial driveways. un- less an adequate radius is included. (See Section 12.0, Driveways.) 5.7 Curb Placement The location of a curb with respect to the traffic lane is important from a traffic operations viewpoint. Most drivers will shy 3 or more feet (1 m) away from a typical 6 to 8 inch (15 to 20 cm) barrier curb on the right unless forced by heavy traffic to ride closer.30 Drivers will drive closer to a barrier curb on their left although this allows them less room to move to the left IO avoid other vehicles. If a mountable curb is used. the lat- eral shift will vary from 6 Inches to 2 feet (0.15 to 0.60 m). depending on