You need to upgrade your Flash Player
Download the most recent Flash plugin here.


The action of reducing the vibration of an object; this tends to return the vibrating object to its original position.
dark adaptation
Adjustment of the eye to low levels of illumination, which results in increased sensitivity to light.
dead load
A static load due to the weight of the structure itself.
The movement of a transit vehicle without fare-paying passengers aboard, often to and from a garage or terminal or between routes, also refers to commercial truck movement when the unit is moving empty cargo.
deceleration lane
A lane designed to allow traffic to safely decrease speed.
decision sight distance (DSD)
The distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered, recognize the hazard or its threat potential, select an appropriate speed and path, and initiate and complete the required safety maneuver safely and efficiently.
The roadway portion of a bridge that directly supports vehicular and pedestrian traffic.
deck bridge
A bridge in which the supporting members are all beneath the roadway.
deck truss
A bridge whose roadway is supported from beneath by a truss.
depth perception
The ability to distinguish the relative distance of objects in visual space, used to interpret their motion over multiple observations.
design criteria
Established state and national standards and procedures that guide the establishment of roadway layouts, alignments, geometry and dimensions for specified types of roadways in certain defined conditions; the principal design criteria for roadways are traffic volume, design speed, the physical characteristics of vehicles, the classification of vehicles and the percentage of various vehicle classification types that use the roadway.
A sloping structural member of a truss or bracing system.
diamond interchange
The simplest and perhaps most common type of interchange; this type of interchange contains a one-way, diamond-type ramp in one or more of the quadrants; the diamond interchange provides for all movements to and from the intersecting road.
disadvantaged business enterprise (DBE)
A business owned and operated by one or more socially and economically disadvantaged individuals; socially and economically disadvantaged individuals include African Americans, Hispanic Americans, Native Americans, Asian Pacific Americans or Asian Indian Americans, and any other minorities or individuals found to be disadvantaged by the Small Business Administration under Section 8(a) of the Small Business Act.
discretionary spending
A federal budgetary term that refers to any funds whose distribution is not automatic; discretionary spending encompasses programs controlled by annual appropriations bills and is subject to the constraints imposed by the discretionary spending limits set in the balanced budget law.
diverge steering zone
Used with interchange/ramp exit models; it is the distance upstream from the exit gore at which a driver begins to diverge from the freeway.
divided attention
The ability of a driver to allocate attention among tasks or stimuli in the roadway environment, where more than one task or stimulus is perceived to be important to safe performance at a given time.
divided highway
Roadway that is separated by a median.
Shipping containers that can be stacked atop one another on a railroad flatcar.
drawn down
Using funds that have been awarded.
dynamic message sign (DMS)
An electronic traffic sign is often used on roadways to give travelers information about special events; they warn of traffic congestion, accidents, incidents, roadwork zones or speed limits on a specific highway segment; they may also ask vehicles to take alternative routes, limit travel speed, warn of duration and location of the incidents, or just inform of the traffic conditions.
dynamic visual acuity
Acuteness or clarity of vision for an object that has angular movement relative to the observer; acuity depends on sharpness of retinal focus, sensitivity of nervous elements, oculomotor coordination, interpretive faculty of the brain and contextual variables.