chainage.C.4Cases of misphasing fall into four types. Theseare described below together with the necessarycorrective action for each type.Insufficient separation between the curves.C.5If there is insufficient separation between theends of the horizontal and vertical curves, a false reversecurve may appear on the outside edge-line at thebeginning of the horizontal curve, or on the inside edge-line at the end of the horizontal curve. This is a visualdefect. It is illustrated in Figs Cl(a) and Cl(b).C.6Corrective action consists of increasing the28separation between the curvesThe vertical curve overlaps one end ofthe horizontal curveC.7If a vertical crest curve overlaps either thebeginning or the end of a horizontal curve, a driver'sperception of the change of direction at the start of thehorizontal curve may be delayed because his sightdistance is reduced by the vertical curve. This defect ishazardous. The position of the crest is important becausevehicles tend to increase speed on the down gradientfollowing the highest point of the crest curve, and thedanger due to an unexpected change of direction isconsequently greater. If a vertical sag curve overlaps ahorizontal curve, an apparent kink may be produced.This visual defect is illustrated in Fig C1(c).C8 The defect may be corrected in both cases bycompletely separating the curves. If this is uneconomic,the curves must be adjusted so that they are coincident atboth ends, if the horizontal curve is of short radius, orthey need be coincident at only one end, if the horizontalcurve is of longer radius.Both ends of the vertical curve lie onthe horizontal curveC.9If both ends of a crest curve lie on a sharphorizontal curve, the radius of the horizontal curve mayappear to the driver to decrease abruptly over the lengthof the crest curve. If the vertical curve is a sag curve, theradius of the horizontal curve may appear to increase. Anexample of such a visual defect is illustrated in FigC1(d). The corrective action is to make both ends of thecurves coincident, or to separate them.The vertical curve overlaps both endsof the horizontal curveC.10 If a vertical crest curve overlaps both ends of asharp horizontal curve, a hazard may be created becausea vehicle has to undergo a sudden change of directionduring passage of the vertical curve while sight distanceis reduced.C.11The corrective action is to make both ends ofthe curves coincident. If the horizontal curve is lesssharp, then a hazard may still be created if the crestoccurs off the horizontal curve because the change ofdirection at the beginning of the horizontal curve willthen occur on a downgrade (for traffic in one direction)where vehicles may be increasing speed.C.12The corrective action is to make the curvescoincident at one end so as to bring the crest on to thehorizontal curve.
C.13No action is necessary if a vertical curve thathas no crest is combined with a gentle horizontal curve.