Annex 3A: Guidelines for Local Gravel Road Rehabilitation 436 Horizontal alignment The horizontal alignment of the road is a series of straight lines, called tangents, connected by curves. Normally, the largest possible radius of curvature should be provided, however, there are limitations and controlling elements of the design that were considered such as limiting cost for earthworks and right-of-way acquisition. Some of the guidelines used to attain effective design of horizontal alignment, are as follows: 1. Avoid compound curves whenever possible 2. Avoid long straight tangents; this will alleviate driver’s fatigue 3. Avoid short tangent length between curves, this will result in poor visual appearance 4. Horizontal curves for small delfection angles should be suiffciently long to avoid the appearance of kink 5. Curves should be at least 150 m long for an intersection angle of ifve (5) degrees, and the minimum length should be increased by 30 m for each 1 degree decrease in angle 6. Minor road widening should increase the carriageway width to the minimum standard width provided that such widening can be carried out within the available RROW. Road widening to increase carriageway width to more than the minimum standard width is considered to be road improvement. 7. Widening of existing road shoulders should be limited to achieving the minimum standard width on each side of the carriageway. Similarly, new road shoulders conforming to the minimum standard width may be added to each side of the carriageway in locations where no road shoulders exist. All shoulder widening and provision must be undertaken within the existing RROW. Providing road shoulders wider than the minimum standard is considered to be road improvement. Vertical alignment The maximum gradient for each class of road is a function of the terrain to be followed. However, the design of gravel roads may sometimes require that the maximum desirable gradient is exceeded particularly in mountainous areas in order to minimize costly realignments and extensive earthworks. A vertical curve is considered adequate if it allows a visibility such that a travelling vehicle can see an obstruction, or another vehicle travelling in the opposite direction, with suiffcient time to react in the proper manner. The major control for safe operation on crest vertical curves is the provision of ample sight distance for the roadway design speed.