Maximum gradients

The higher the road is in the hierarchy of the road network, the flatter its maximum gradient should be. Streets in an urban township can be extremely steep and developers, whose sole aim is to create as many saleable properties as possible, will have no compunction in propos- ing gradients steeper than the ability of passenger cars to climb them.

San Francisco is well known for its steep streets. Lombard Street is internationally famous and best known for the one-way section on Russian Hill between Hyde and Leavenworth Streets, in which the roadway has eight sharp turns (or switchbacks) that have earned the street the distinction of being the most crooked street in the world. The switchback design, instituted in 1922, was born out of necessity to reduce the hill’s natural 27 per cent grade, which, at that time, was too steep for most vehicles to climb. The crooked section of the street, which is about 1⁄4 mile (400 m) long and reserved for one-way traffic downhill, is illustrated in Figure 7.5.

It can be observed that parking on the section of Lombard Street below the switchback is at right angles to the centreline of the street to avoid the possibility of parking brakes not being able to hold vehicles on the downgrade.

The two steepest streets in San Francisco are Filbert Street, between Hyde and Leavenworth Streets, and 22nd Street, between Church and Vicksburg Streets. They have grades of 31.5 per cent. Both of these streets are one-way, going downhill.

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Some older streets in the United States as listed below have gradients steeper than 30 per cent (http://www.geographylists.com/list17y.html). These are

1. Honokaa-Waipio Road (near Waipio, HI, maximum grade 45 per cent) 2. Canton Avenue (between Coast and Hampshire, Pittsburgh, PA, 37 per cent) 3. 28th Street (between Gaffey and Peck, Los Angeles, CA, 33.3 per cent) 4. Eldred Street (west of Avenue 48, Los Angeles, CA, 33 per cent)

5. Baxter Street (between Alvarado and Allesandro, Los Angeles, CA, 32 per cent) 6. Fargo Street (between Alvarado and Allesandro, Los Angeles, CA, 32 per cent) 7. Maria Avenue (north of Chestnut, Spring Valley [near San Diego], CA, 32 per cent) 8. Dornbush Street (between Bricelyn and Vidette, Pittsburgh, PA, 31.98 per cent)

7.4.2.1 Desirable maximum gradients

Most urban street authorities now will not permit gradients steeper than 15 per cent. The United Kingdom specifies that any gradient steeper than 8 per cent will be considered to be a design exception requiring the issue of a waiver by the road authority (Highways Agency et al., 2002). Construction problems require that roads steeper than about 10 to 11 per cent should be concrete paved. Twelve-fourteen–tonne rollers are at their limits of hill-climbing ability on these gradients, so that attempts to compact the base course using these rollers would most likely be unsuccessful. They also tend to create the ‘rumpled tablecloth effect’, building corrugations into the road surface on stopping at the bottom of their run.

Operationally, braking on a steep downgrade on a bitumen surface will create the same effect. Truck drivers also experience significant difficulties in pulling away from rest on upgrades this steep. Typical maximum gradients are shown in Table 7.1.

In keeping with the philosophy of providing guidelines as opposed to rigid applied stan- dards, it follows that there may be sound reason for exceeding these maximum gradients. These reasons may include

• Extremely adverse topography where gradients flatter than the maximum are not read- ily achievable without considerable outlay in construction costs

• Areas where short lengths of steeper gradient can achieve considerable cost savings • Low volumes of truck traffic suggesting that an impediment to the free movement of

passenger cars and other lighter vehicles is not an issue

The maximum gradients should not be adopted as a general rule and, preferably, should be used as sparingly as possible. If truck volumes exceed about 5 per cent, it may be useful to consider the provision of climbing lanes on steep gradients.

Table 7.1 Maximum gradients Design speed

(km/h)

Gradients (%), by topography

Flat Rolling Mountainous

60 6 7 8

80 5 6 7

100 4 5 6

120 3 4 5

Source: Burrell RC et al., Geometric design guidelines. South African National Road Agency Limited (SANRAL), Pretoria, 2002.

Vertical alignment 129

7.4.2.2 Safety on steep downgrades

Steep upgrades mean steep downgrades in the opposite direction, and sensible truck drivers tend to essay these downgrades at about the same speed that they can achieve on the up- grade. Passing a slow-moving truck on a steep downgrade would normally not be a problem but, if passing opportunities are restricted, it may be worthwhile to consider a ‘descending’ lane. In addition, arrestor beds may be used to address the problem of runaway trucks. As a final safety measure, compulsory stops at the start of a long downgrade provide an opportunity to cool the brakes before the downgrade is essayed. A problem that truck driv- ers sometimes experience is a missed gear change whereby it is found that, having changed into neutral, it is not possible to engage the next gear. The truck then freewheels down the downgrade with a distinct possibility of loss of control and the resultant crash. The compul- sory stop makes it possible to select the appropriate gear prior to the descent. Fortunately, the general adoption of automatic transmissions on trucks is phasing this problem out but there are still numerous trucks on the world’s highways that have crash gearboxes, that is, transmissions without the benefit of synchromesh where the missed gear change is still a distinct possibility.

Speeds increase automatically on long downgrades and it is prudent to attempt to increase the value of geometric standards progressively to match the higher operating speeds likely to be found towards the bottom of the grade.

Steep downgrades can create problems with regard to storm water drainage in urban areas. Highly energised streams of water simply flow past drop inlets and ultimately flow across the surfaces of intersecting streets.

In rural areas, the combination of camber and a steep gradient may result in water flowing for a long distance down the road before achieving the shoulder. The possibility of vehicles hydroplaning is an ever-present danger.

Foul water sewers are usually provided within the road reserve and thus tend to have gradients similar to that of the road. On steep gradients, separation between fluid and solid waste can occur, leading to blockage of the drain.

7.4.2.3 Maximum gradients on gravel roads

Gravel roads are subject to scour so that gradients of more than about 6 per cent should be avoided. Where this is not possible, the grades should be paved. Paving should be provided at least from the point at which the gradient exceeds 6 percent to the point at which the gradient drops back to 6 per cent. Because the roughness coefficient of concrete is lower than that of gravel, flow down the paved section would be faster than that on a gravel sur- face with a gradient of 6 per cent. The downstream end of the paved section should thus be extended to a point where the gradient is such that the flow speed of the water would be that associated with a 6 per cent gradient on a gravel surface. As a further measure, the paved section should be provided with a camber or crossfall redirecting the flow of water to a side drain. The side drain should also be paved as a protection against scour on the steep gradients.

Roads are usually gravelled when traffic volumes are not high enough to warrant paving. Bitumen surfaces require the kneading action provided by traffic to maintain their flexibil- ity. Insufficient traffic, for example, volumes of less than about 400 vehicles per day, thus suggests that paving should be concrete. It is possible that storm water flowing towards a paved section could undercut it, creating a dangerous situation. The upstream terminal of the paved section should thus be provided with a cutoff wall. Water flowing from the paved section onto the downstream gravel could be moving swiftly. Protection against scour will

130 Geometric design of roads handbook

have to be provided either by directing the flow to the side of the road by the provision of a camber or straight crossfall or by extending the paved section to where the gradient is flat enough to cause the flow to be slower than that likely to cause scour.