There are many factors affecting safety on local streets and highways, and one of these is how drainage—the run-off of water from the pavement and shoulder—is treated.
The primary objective in treating drainage is to ensure that safe operating conditions exist on the roadway. This is done by removing storm run-off, providing for snow removal and reducing the ability for ice to form on roadways and bridges. Additionally, the drainage systems used on the roadway should not be hazardous or reduce the crashworthiness of other systems, such as breakaway signs and guardrail.
Also included as part of this objective is the safe design and maintenance of drainage features, such as travel way and shoulder surfaces, side slopes, drop inlets, pipe ends, ditches, headwalls, safety grates, culverts, gutters, roadside and median barriers and other features that restrict the flow of water or removal of snow.
Travel Way Pavement Surface
Drainage factors that affect safety on the travel way of streets and highways include:
- Even a thin layer of water on the travel way surface can initiate motor vehicle hydroplaning at speeds as low as 35 MPH. When a vehicle tire rides on top of this thin layer of water, the vehicle cannot be steered or stopped easily. A vehicle that is hydroplaning behaves similarly to a vehicle on ice.
- Water ponding(2) in the wheel paths or ruts is also very dangerous, particularly for motorcycles and bicycles. Water ponding at intersections may be especially hazardous as vehicles may require greater distances to stop.
(2) Ponding refers to water collecting on the pavement or shoulder that is deteriorated. Standing water refers to water collecting on the surface due to insufficient cross slope or backup in the drainage system.
Travel ways are generally designed to provide sufficient cross slope to facilitate storm run-off; however both traffic and maintenance activities can and often do affect the original cross slope of the roadway surface. Traffic, particularly heavy truck traffic, can cause wheel ruts in the pavement surface.
Rutting is caused by the wear or movement of the bituminous pavement to the sides of the wheel path. Rutting usually runs parallel to the centerline of the roadway. When wheel ruts are noticeable, they are generally deep enough to pond sufficient water to initiate hydroplaning.
Another pavement condition, often referred to as shoving, can occur where vehicles frequently stop, such as at intersections. Shoving is most common at stop locations, particularly on steep downgrades. Shoving results in a series of ridges running across the roadway. The ridges can retain water and are hazardous even when dry because they cause the wheels of a vehicle to bounce, reducing wheel contact with the roadway surface and decreasing braking ability.
Ponding or standing water in the travel way may also cause some drivers, bicyclists, or pedestrians to divert from their desired path. Storm drains or drop inlets that have collected debris, such as leaves, can cause ponding during and after storms. Both motor vehicle and bicycle operators may choose to avoid going through the water by encroaching on the opposing lane, thus endangering themselves and opposing traffic.
In sections of the travel way where run-off drains directly onto the shoulders (where there are no curb and/or gutters), water may collect along the edge of the travel way. Water on a portion of roadway can result in drivers losing control of their motor vehicle, particularly when braking in an emergency. This can happen when the inside tires are in contact with roadway surface while the braking ability of the outside tires is hindered by the water.
Water can pond on the outside edge of the travel way surface when debris, particularly aggregate and soil on turf shoulders, builds up. As debris accumulates on the shoulder, it raises the level of the edge, and eventually hinders run-off from flowing into side ditches.
Water ponding on the edge of the pavement contributes to the deterioration of the pavement edge and the rutting of stabilized soil supporting the pavement edge, which can result in additional safety hazards. Edge drop-offs and shoulder scour are often caused when water is trapped at the pavement edge by the build-up of debris and vegetation growth.
Partial overlays and pavement repairs can result in water being trapped and retained on the travel way surface. Partial overlays, either to correct shoulder deterioration or widen the roadway surface, result in a pavement edge where the overlay stops. Depending on the size of aggregate in the overlay mix and the effort taken to feather the lip into the existing pavement, water can be retained on the travel way. When the lip is along the wheel path, the thin layer of retained water can initiate hydroplaning, reduce braking ability or freeze and contribute to skidding.
Ditches and Side Slopes
The area along the side of the roadway is also affected by run-off. Ditches and side slopes (either foreslope or backslope) are affected by storm run-off and maintenance activities. Motor vehicles and bicycles occasionally run off the roadway, but when the side slopes are relatively flat (1 vertical:10 horizontal [1V:10H] slope or flatter) and well graded (free of erosion scars) and ditches maintained with traversable side slopes, most drivers can recover control of the vehicle and return to the roadway or come to a safe stop.
One of the reasons that run-off-the-road errant vehicles may not be involved in a severe crash is safer shoulders and forgiving roadsides. Many roadways are designed to allow run-off to travel across the shoulders and be collected in a ditch or channel or flow down the side slope and seek a natural channel.
Drainage ditches are common on roadways when curbs are not used to channelize storm run-off. There are also cut-off ditches used along the travel way to prevent run-off from adjacent land flowing onto the travel way.
Drainage ditches should be designed so a vehicle leaving the roadway can cross over them without the vehicle overturning, being abruptly stopped or causing the driver to lose control.
Drainage ditches over which a motor vehicle can safely drive are called traversable. Design criteria for traversable drainage ditches are provided in the current edition of the Roadside Design Guide (see references at the end of this guide).
Ditches need to be cleaned on a regular basis to prevent them from silting up and forcing water back onto the travel way surface or into the subbase of the pavement.
Earth ditches are subject to erosion. The ruts and deposits of silt and debris can change the shape of the ditch, resulting in sections that are no longer traversable or forgiving. Additionally, maintenance activities, such as cleaning ditches, can result in creating a non-traversable ditch.
Side slopes along the travel way may also become eroded, particularly when ditches silt up. The steeper side slopes (1V:3H and 1V:4H) that are marginally traversable become hazardous when they develop erosion channels that can trip or snag a tire.
In addition to ditches and side slopes, incorrectly maintained drop inlets, pipe ends, culvert ends, head walls, and other drainage features located adjacent to the roadway may be potentially hazardous.
Pipe ends, culvert wing walls, and headwalls adjacent to the roadway are potentially hazardous when they extend above the surrounding ground. These features are rigid objects that can snag the undercarriage of a vehicle leaving the roadway or initiate vehicle rollover.
Many of the older drainage features were built above the ground. These features are rigid objects that can cause serious injury if hit. Current practice provides for traversable ends, extension or relocation of the ends far enough away from the roadway to reduce the risk of a crash or use of a barrier system to shield road users from a hazardous location.
Even traversable drainage features can become hazards when storm run-off or maintenance operations, such as dressing side slopes, result in the feature extending above the surrounding ground. These features are rigid objects that can snag the undercarriage of a vehicle leaving the roadway or initiate vehicle rollover.
Pedestrian and Bicycle Paths
The safety of pedestrians and bicyclists can also be jeopardized by poor drainage in the travel way, shoulder, or paths. While these users are more able to avoid or compensate for standing water or ice in their path, maintenance activities and relatively low-cost improvements can prevent drainage problems and improve the safety for these users.