Chapter 3: Elements of Design Offtracking (p.3-85 – p.3-97) Be able to explain the components of the offtracking equation, U, C, F A, Z Be able to determine these values for selected design vehicles and determine design values for widening on horizontal curves Objectives: 1

Offtracking Offtracking is the characteristics, common to all vehicles, although much more pronounced with the larger design vehicles, in which the rear wheels do not follow precisely the same path as the front wheels when the vehicle negotiates a horizontal curve or makes a turn. 2 Fig. 2-10

u (normal track width) = 8.5 ft for WB-62 3

Derivation of Design Values for Widening on H-curves: U, C, F A, F B, and Z U is the track width that includes off- tracking effect. [Eq. 3-31] u = normal track width R = turning radius of the outer front wheel. L i = wheelbase L Use of the Pythagorean theorem. u For an articulated vehicle, each of the articulation points is used to determine U. For example, a tractor/semitrailer combination truck has three L i values that are considered: (1) the distance from the front axle to the tractor drive axle(s), (2) from the tractor drive axle(s) to the fifth wheel pivot [the kingpin], and (3) the distance from the fifth wheel pivot to the rear axle(s). P Offtracking value 4

U, C, F A, Z 5 U = Track with of design vehicle (out-to- out tires) C = Lateral clearance F A = Width of front overhand of inner-lane vehicle Z = Extra width allowance

Track width on Curves, U 6 Fig. 3-17

Eq in p A = front overhang of inner lane vehicle, ft; L = wheelbase of single unit or tractor, ft. R T = radius of curve or turn In the case of tractor-trailer combinations, only the wheelbase of the tractor unit is used for determining F a. 7 Fig. 3-18

Traveled way widening on H- curves (p.3-91) (considering offtracking for multiple vehicles) With 12-ft lanes and high-type alignment, the need for widening has lessened considerably in spite of high speeds, but for some conditions of speed, curvature, and width it remains appropriate to widen traveled ways. 8 (3-34)

Lateral clearance (C) and allowance for the difficulty of driving on curves (Z) Z is an empirical values for the extra width allowance to compensate for the difficulty in maneuvering. v = design speed, mph. R = radius in ft [Eq. 3-33]. Extra distance between opposing vehicles (only one Z is added. See Eq. 3-35). Z is an empirical values for the extra width allowance to compensate for the difficulty in maneuvering. v = design speed, mph. R = radius in ft [Eq. 3-33]. Extra distance between opposing vehicles (only one Z is added. See Eq. 3-35). E.g. Z = 60 mph/sqrt(1000 ft) = 1.9 ft E.g. Z = 60 mph/sqrt(1000 ft) = 1.9 ft C provides for the clearance between the edge of the traveled way and nearest wheel path and for the body clearance between vehicles passing or meeting. C = 2, 2.5, and 3 ft for W n = 20, 22, and 24 ft). (note: there is no chart for C.) 9

Z and C 10 Fig. 3-19

Curve Width (Wc) The WB-62 design vehicle is considered representative for two-lane open-highway conditions in GB (3-35)

GB2011 Table 3-26b 12

Application of Widening on Curves (p.3-95) On simple (unspiraled) curves, widening should be applied on the inside edge of the traveled way OR divided equally on either side of the center line. On spiraled curves, widening may be applied on the inside edge or divided equally on either side of the centerline. Preferably, widening should transition over the superelevation runoff length. From the standpoint of usefulness and appearance, the edge of the traveled way through the widening transition should be a smooth, graceful curve. On highway alignment without spirals, smooth and fitting alignment results from attaining widening one-half to two-thirds of the transition length along the tangent and the balance along the curve. PC 13

GB2011 Table 3-27 adjustment values 14 WB62 was used to create Table 3-26b.