ARTICULO 163.- DOMINIO ADMINISTRACION EN LA SEPARACION DE BIENES

The preceding discussion of horizontal curvature has tacitly dealt with isolated curves, that is, remote from each other and with no interaction between them. Interaction could be visu- ally or physically. Visual interaction arises when two curves are visible to the approaching driver at the same time. Physical interaction arises from the curves being so close to each other that normal superelevation development is compromised.

Three combinations of successive curves are possible.

• The reverse or S-curve where the two curves are in the opposite directions

• The broken-back or flat-back curve where the successive curves are in the same direction • The compound curve where the successive curves abut each other and are in the same

direction but with differing radii

The reverse curve matches driver expectations, which are that a curve to the left will be followed by a curve to the right or vice versa. In terms of the merits of curvilinear align- ment, the reverse curve is aesthetically pleasing, particularly so if the radii of the successive curves are similar. By way of contrast, the same cannot be said of the broken-back curve. It is generally considered to be an eyesore of note. It has been said of the broken-back curve that the designer didn’t go around enough the first time! Unfortunately, topographic or other restraints may make it impossible to avoid the use of a broken back curve but a certain amount of effort put into its avoidance is to be encouraged. The flexibility of the compound curve may tempt the designer into using it to negotiate broken terrain. The temptation should, however, be firmly resisted. The human factors approach indicates that, once drivers enter a curve, they expect that its radius will remain unaltered across the length of the curve.

There is only one place where driver expectations support a compound curve and that is on the loop ramps at interchanges. Compound curves may also find application on turning roadways at at-grade intersections but, other than possibly noting that maintaining position on the turning roadway is comfortable, drivers would probably be unaware of the existence of the compound curvature of the roadway.

116 Geometric design of roads handbook

Note:

L1 and L2 are the lengths of superelevation development With transition Without transition 2% 6% 6% 6% 6% 2% 1_/3L1 1_/3L2 EC1 ST1 CS1 TS2 SC2 BC2 Circular

curve 1 Circularcurve 2

Transition

curve 1 Tangent Transitioncurve 2 Tangent

Figure 6.9 Superelevation development in a broken-back curve. (From Burrell RC et al., Geometric design guidelines. South African National Road Agency Limited [SANRAL]. Pretoria, 2002.)

Note:

L1 and L2 are the lengths of superelevation development With transition Without transition L1 e1% e1% e2% e2% 2% 2% 1_/3L1 2_/3L1 2_/3L2 1_/3L2 L2 EC1 ST1 CS1 SC2 TS2 BC2 Tangent Circular

curve 1 _Transition Circularcurve 2

curve 1 Tangent Transitioncurve 2

Figure 6.8 Superelevation development in a reverse curve. (From Burrell RC et al., Geometric design guide­ lines. South African National Road Agency Limited [SANRAL]. Pretoria, 2002.)

Horizontal alignment 117

Failure is a hen and egg phenomenon. In other engineering disciplines, failure is a mea- surable quality responsive to the laws of physics, for example, a beam failing under load. In geometric design, there is a tendency similarly for road users to be dealt with as inanimate matter, that is, as if it were possible to represent them by fixed parameters. In fact, drivers respond according to what they see as modified by historic expectancy. Numerous examples abound. Locating the off-ramp at an interchange on the right-hand lane constitutes practical design. Drivers have now come to expect to see the off-ramp to their right. Designing a loop ramp as a spiral makes it possible to reduce substantially the area required to accommo- date it. Drivers now expect to enter a loop ramp and decelerate to the speed of the smallest radius, accelerating thereafter towards the design speed of the next through road. And this is the only situation in which the use of a compound curve can be entertained.

The compound curve is, in fact, a surrogate for the spiral and came into existence purely to avoid the inconvenience of setting it out. The compound curve is discussed in detail in Chapter 12.

The reverse and the broken-back curves comprise two curves with a short length of tangent between them. The question is one of determining how short the tangent can become without negatively impacting on the superelevation development. It is suggested that the shortest tangent is the one that makes it possible to roll the superelevation over from that of the first curve to that of the second curve in a continuous fashion as illustrated in Figure 6.8 applying the appropriate relative slope. As shown, the relative slope remains constant from end to end of the roll over.

In the case of the broken-back curve, it is suggested that the superelevation runoff ter- minate at the point where a straight crossfall of reverse camber is achieved, as shown in Figure 6.9. The term broken-back is usually not applied if the intervening tangent is of a reasonable length, say about 500 metres. Even then, if both curves are simultaneously visible from some distance ahead, the general appearance is not good.

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Chapter 7