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The density of luminous flux (rate of emission of luminous energy flow of a light source in all directions) incident on a surface; the quotient of the flux divided by the area of the surface, when the surface is uniformly illuminated.
The amount of light falling onto a surface.
An incident may be any of the following: traffic accident, stalled vehicle, load spillage or other action that affects one or more lanes of traffic; an "accident" typically involves a collision of a moving vehicle with another vehicle, person or object.
initial acceleration (IA) zone
Used with interchange/ramp entry traffic models, it is the zone in which the driver accelerates to reduce the speed differential between the ramp vehicle and the freeway vehicle to an acceptable level for completing the merge process.
in-service brightness level (ISBL)
The brightness level of a delineation treatment at an intermediate point in its anticipated service life; this value varies by type of delineator, type of wear (traffic level) and environmental conditions.
intelligent transportation system (ITS)
ITS is a surveillance system designed to monitor traffic flows on major freeways and to inform motorists of problem areas. Some ITS technology includes changeable message signs (CMS), cameras, and video detectors. ITS applications include the integration of traffic control and transportation management systems; traffic signals which adapt to traffic and change control each cycle; highway advisory radio systems; CMS; vehicle detectors; closed-circuit television; global positioning systems and route guidance (currently used in some trucks, buses and rental cars). An ITS may enable drivers to operate their vehicles more safely and with greater knowledge about existing traffic conditions.
interchange (grade separation)
A system of interconnecting roadways that provides for the movement of traffic between two or more highways on different levels.
Switching from one form of transportation to another.
intermodal facility
A transportation element that accommodates and interconnects different modes of transportation and services the statewide, interstate and international movement of people or goods. For example, an intermodal yard is a railyard that facilitates the transfer of containers or trailers to a commercial motor carrier.
intermodal service
Freight moving via at least two different modes of transport; intermodal service generally involves the shipment of containers and trailers by rail, truck, barge or ship.
International Registration Plan (IRP)
IRP is a method of registering fleets of vehicles that travel in two or more member jurisdictions. Motor carriers that qualify for IRP must register fleets of vehicles in their home or “base” jurisdiction. A fleet, for IRP purposes, is comprised of one or more vehicles that pay registration fees in multiple states.
intersecting angle (skew)
The angle formed by the intersection of two roadways (other than a 90-degree angle).
intersection sight distance (ISD)
The unobstructed view of an entire (at-grade) intersection and sufficient lengths of the intersecting highway to permit control of the vehicle to avoid collisions during through and turning movements.
Interstate Highway System
A network of freeways in the United States; the Interstate Highway System is a separate system within the larger National Highway System; the entire system, as of October 2002, had a total length of 46,726 miles.
Iowa in Motion
The process used by the department to develop the State Transportation Plan (STP) that was adopted by the Commission on July 15, 1997; the plan is intended to guide the development of transportation policies, goals, objectives, initiatives and investment decisions through the year 2020.
Iowa Plane Coordinate System
Iowa's geographical coordinate system.
Iowa Statewide Transportation Improvement Program (STIP)
A two-year program of transportation projects required by SAFETEA-LU, consisting of a federally required document that allocates transportation funds to a staged, multiyear, statewide, intermodal program of transportation projects; consistent with the statewide transportation plan and planning processes and metropolitan plans, TIPs and processes; the metropolitan TIP must be included in the STIP without change.