There are several designs for highways and roadways. These designs require a lot of engineering as there are many factors that are considered in the design of highways. The most important fact considered is the expected average speed of the vehicles that will be passing through the highways. In other words, there are different types of road designs for different types of highways and roadways.
Geometrical design of roads is a branch of road engineering that focuses on the geometrical features of the roadway. Geometric designs involve the design features of roadways and highways especially the vertical and horizontal alignment of the road. Even though geometric designs have several purposes, the most important purpose of geometric designs is the provision of seamless traffic operations. As mentioned earlier, speed of vehicle is a big determining factor of the geometric designs of highways and roadways.
Geometrical roadway design can be divided into three main parts – cross-section, profile and alignment. The cross-section shows the position and number of sidewalks, bicycle lanes and vehicle lanes with some banking. Cross-section also covers pavements and drainages. The profile of a road focuses on the vertical aspect on the road. The profile of a road includes sag curves, crest and the grade lines that connect them. The alignment of any roadway is its route. It is defined by some horizontal tangents and curves.
Design speed is the speed chosen by the engineers for which each road is designed. The engineers will assume the design speed as the maximum speed of the vehicles that will be passing through the road on a daily basis. It is also important to note that design speed is not considered alone. Instead, it is considered with a particular number of vehicles. For instance, when designing a road, engineers can assume a daily average number of vehicles as 200 at the design speed of 60 km/hr. However, it is worthy of note that the speed should be set as maximum to be able to offer maximum satisfaction with drivers.
However, there is a laid down standard for design speed and expected traffic. The design speed is determined by the nature of the terrain. For this purpose, roadways and highways have been grouped into different category of terrains – Flat terrain, rolling terrain, mountainous terrain. Another road categorization has also been done with regards to how busy the road is. There are just two road categories for this – High volume roads and low volume roads.
The design speeds for high volume roads
The laid down design speed for flat terrain is 45 km/h for a maximum of 250 vehicles in a day. For a road that has a daily traffic of 251 to 400 vehicles, the design speed is 60 km/hr. The design speed of 75 km/hr is meant roads with traffic of more than 400 vehicles.
If it is a rolling terrain road, then the design speed should be as follows: 30 km/hr for a road whose daily traffic is less than 50 vehicles. It is 45 km/hr for a road whose daily traffic is between 50 and 400 vehicles. The design speed of 60 km/hr is the best for roads whose daily traffic is above 400 vehicles.
Mountainous roads have the least set of design speeds. It is 30 km/hr for roads whose daily traffic is 400 vehicles or less and 45km/hr for roads whose daily traffic is above 400 vehicles.
The Design speeds for low volume roads
For low volume roads, the design speeds are as follows:
- 45 km/hr for flat terrain roads whose daily traffic is 50 or less
- 30 km/hr for rolling terrain roads
- 30 km/hr for mountainous roads
Road alignment is important at the intersection of a road. It is dangerous for a road to change from straight and flat to a very sharp curve. Apart from making it uncomfortable for drivers, it also attracts accidents as vehicles can skid or veer of the road if handled by an inexperienced or a careless driver.
Road alignment should be done in such a way that the road gradually changes from a straight to a sharp curve. Road alignment is made of two components which are vertical and horizontal alignment. Every road should have a certain degree of curve and this curve is determined by working out the best radius for the road. When the degree of curve is right, drivers will not feel the sudden change of course while approaching the bend. This gives motorists wonderful experience.
This curve is based on the horizontal alignment. The horizontal profile of a road is the most important factor of the horizontal alignment. The design speed of the road is also considered while designing the road curve. There is a particular formula used to determine the best radius for a road. The formula makes use of the following components. The vehicle speed, the rate of roadway super elevation (this is usually in percentage) and the side friction factor. All of them are used to calculate the radius in feet.
Vertical alignment is also important. Vertical alignment curve is also provided during the design stage and it is done on the profile of a roadway. The importance of vertical alignment is felt when a vehicle is moving either uphill or downhill. It ensures that the slope is not steep so that the vehicle does not feel the steepness. The steeper a road is, the more difficult it is to climb. This is why the vertical alignment is very important.
Roadway width and superelevation
Roadway width is another important feature to consider. Design speed of a roadway increases proportionally with its width. This is quite logical. As a driver, the wider a road is, determines how fast you can move. People usually slow down when they get to narrow roads. On narrow roads, a little swerve can cause a terrible accident. This is because a wider roadway has more room for drivers to maneuver when anything suddenly goes wrong. Drivers are meant to slow down once they begin to approach a narrow road of a narrow part of a major roadway.
Superelevation is another factor that is worthy of mention. When a vehicle is approaching a curve, the elevation of left part of the road will be slightly higher than that of the right part or the reverse depending on which part the road is curved towards and the topography of the road. This actually counters the centrifugal force exerted by the navigating road. This feature is known as either road cambering or superelevation.
Road design engineers also consider sight distance. Sight distance is simply how far a driver can see ahead. Depending on the speed of a vehicle, drivers are not usually able to avoid an object that is too close. And if they do, they usually veer off the road. This is why it is better to have seen the obstruction a good distance ahead. The long the sight distance of a road the safer it is especially if it’s a dual lane road where a driver has to be sure that there is no oncoming vehicle so that he can overtake another vehicle.
Every road must have a sufficient sight distance when coming from both ends. Even though it is a safety principle to maintain a low speed on roads whose sight distance are very short, every road engineer must ensure he gives a road a very long sight distance. The main objective of all these factors is to ensure maximum comfort and safety on roadways and highways.
There are several types of sight distance. These types of sight distance are stopping sight distance, decision sight distance, intersection sight distance and corner sight distance. The stopping sight distance is distance moved from the point a driver decides to stop and the point at which the vehicle stops. This is necessary because the vehicle cannot just stop, it has to decelerate gradually before it finally stops. A lot of things could have made the driver decide to stop. It could an obstruction, perceived fault in the car, sighting of an acquaintance or poor road condition. While designing the stopping sight distance, the design speed is considered so that any driver travelling at the design speed or less will have enough stopping sight distance.
Decision sight distance is a little longer and more complex to explain than stopping sight distance. It is the distance given to a driver for him to decide on taking more complex decision other than stopping. The decision sight distance covers decision, deceleration and successful safe maneuver. One of the common reasons why drivers try to maneuver suddenly is when they another motorist suddenly swerves dangerously. According the common decision sight distance standards, it is assumed that it should take a driver between 6 to 10 seconds from the point of deciding to maneuver to the beginning of deceleration. And it should also take between 4 to 5 seconds to complete a safe maneuver.
Intersection sight distance is simply the sight distance required to for a driver to sight an intersection ahead. However, it is not only the engineer that determines this. Intersection sight distance is also determined by the type of traffic control at the intersection.
Corner sight distance is the distance required by a motorist, pedestrian or cyclist at crossroad to see an oncoming vehicle and still be able to cross with the driver of the oncoming vehicle applying break or slowing down. A lot of roads have insufficient corner sight distance. So, by the time they sight an oncoming vehicle, it will be too late to cross the road.
The point where several roads meet is called an intersection. It is the meeting point of two or more roads. It is the part of the road that is most prone to accidents. The number of roads that meet in an intersection is directly proportional to the level danger in it. This is why intersection safety is of utmost concern to several road traffic organizations. In fact, according to statistics, one quarter of fatal road accidents that occurred within the last few years in the United States have been linked to intersections.
In the United States, intersections are known as conflict points. Motorists, pedestrians and pedestrians meet at this conflict points to cross paths and continue their journeys or turn to a new route or direction. Sometimes during high volume traffic, intersections become too congested and this usually results in delay, frustrations and impatience.
There are several safety efforts that are aimed at reducing accidents at intersections and to also promote safety. Some of these strategies are incorporated during the design of each road. Some of them are geometrical designs that have been discussed above and traffic control devices. The most common devices are markings, signs and signals. Due to the disturbing number of accidents that are related to intersections, the safety of intersections has become an important issue in so many countries. There are two major types of intersections – G-Separated intersections and At-grade intersections.
Pavement Materials and designs
There are two major types of pavements. There are rigid pavements and flexible pavements. Flexible pavements are the ones whose surface is made with either asphalt or bitumen. The structure of flexible pavements usually deflects or bends when there are high traffic loads. The structure of a flexible pavement is usually made of several layers so that it can withstand the bends.
A flexible pavement design consists of either three or four layers. These layers are surface course, base course, sub-base course and these layers are usually constructed over a natural soil subgrade. On the other hand, rigid pavements are usually made of just three layers. These layers are a prepared subgrade, base or sub-base course and a concrete slab. This design is best for location with high volume traffic like the airport terminal. Rigid pavements usually last for about 30 to 40 years.
Rigid pavements on the other hand are made of PCC surface course and this makes them stiffer than flexible pavements. Concrete and asphalt are two main pavement materials so it is necessary to see the differences between the two of them.
Pavements made with asphalt require more maintenance than pavements made with concrete. This means that concrete offers less cost of maintenance. Apart from that roads with concrete pavements will offer more seamless flow of traffic as you would hardly see repair crews working on those roads. Rigid concrete is also more durable than asphalt. So, concrete pavements will definitely last longer. Roads with concrete are less prone to vehicle skidding and veering off the road. That makes roads with concrete pavements a little safer than the ones with asphalt pavements.
General information for road designs
While designing highways, several performance, service and safety standards must be met. In designing highway geometry, social and environmental effects are considered. For a highway to be successfully designed, the following should be put into consideration as they determine the success of any highway design. The design speed of the highway should be considered and that determines on the terrain of the highway. The design traffic volume matters too. You need to consider the daily traffic of a road. The number of lanes of the highway should be considered as it is an important factor too.
It is also necessary to consider the line of service (LOS), all the sight distances that have been discussed above, vertical and horizontal clearance, lane width, cross-section, grades, super-elevation and alignment. The operational performance of any highway can be assessed only by driving through the road. While driving through the road, observations are made and recommendations are also made.