Driving on the Right or on the Left

In India, driving is on the left side of the road.

Traffic drives on the right or on the left side of the road depending on the country. In countries where traffic drives on the right, traffic signs are mostly on the right side of the road, roundabouts and traffic circles go counter-clockwise, and pedestrians crossing a two-way road should watch out for traffic from the left first. In countries where traffic drives on the left, the reverse is true. Traffic flow and road design in both cases are each other’s mirror image.

Construction

A major road near Sibiu, Romania.

Road construction requires the creation of a continuous right-of-way, overcoming geographic obstacles and having grades low enough to permit vehicle or foot travel. Removal of earth and rock by digging or blasting, construction of embankments, bridges and tunnels, and removal of vegetation (this may involve deforestation) are often needed. A variety of road building equipment is employed in road building.

Once these activities are completed, construction of the pavement can begin. First the native soil, known as the subgrade, is compacted. Weak soils may also be stabilized with additives such as portland cement and quicklime, or dug out and replaced with imported soils.

Then a base course consisting of gravel or crushed stone is usually placed on the subgrade and compacted. On top of the base course is placed a surface course which typically consists of asphalt concrete or portland cement concrete. This surface course strengthens the pavement structure by spreading out the vehicle loads applied to the subgrade. It also provides a smooth and high-friction surface for vehicles to drive on.

Modern roads, and indeed many ancient ones, such as those built by the Romans, feature a convex transverse profile known as superelevation or camber. This is designed to allow water to drain away from the road to its edges. Water is then carried away by gutters to drains placed at intervals. Some roads don't have gutters and water simply drains away to a naturally porous verge, or into ditches. Modern roads that carry motor traffic also employ camber in curves to aid traffic stability by allowing them to “bank into” the bend to some extent.

On the side of the road there may be retroreflectors on pegs, rocks or crash barriers, white toward the direction of the traffic on that side of the road, and red toward the other direction. In the road surface there may be cat’s eyes: retroreflectors that protrude slightly, but which can be driven over without damage.

Road signs are often also made retroreflective or even illuminated in rare circumstances. For greater visibility of road signs at daytime, sometimes fluorescence is applied to get very bright colors.

Maintenance

A typical rural county road in Indiana, USA, where traffic drives on the right. The yellow lines indicate that passing is allowed in the ongoing direction but not in the oncoming direction.

Like all structures, roads deteriorate over time. Deterioration is primarily due to accumulated damage from vehicles; however environmental effects such as frost heaves, thermal cracking and oxidation often contribute. According to a series of experiments carried out in the late 1950s it was empirically determined that the effective damage done to the road is roughly proportional to the 4th power of axle weight. A typical tractor-trailer weighing 80,000 pounds with 8,000 pounds on the steer axle and 36,000 pounds on both of the tandem axle groups is expected to do 7,800 times more damage than a passenger vehicle with 2,000 pounds on each axle. In most pavement design methodologies trucks are considered to be the sole cause of pavement deterioration.

Pavements are designed for an expected service life. Most European countries have strict standards for road construction that require that most roads should be able to go 30 years or longer between major resurfacings. In the United States new pavements are typically designed for a service life of between 15 and 25 years, depending on the importance of the road. Service life predictions are inherently unreliable due to the difficulty of predicting future traffic and environmental conditions.