A system, positioned at a site along a roadway, for determining the concentration of one or more constituents present in a sample of exhaust emissions emitted by a vehicle traveling on the roadway, the system comprising: a component detection system configured to determine the concentration of one or more constituents present in a sample of vehicle exhaust emissions; a first collector positioned above the surface of the roadway, the first collector having one or more openings configured to receive exhaust emissions emitted by vehicles in an upward direction or at an elevated position; a second collector positioned on or near the surface of the roadway, the second collector having one or more openings configured to receive exhaust emissions emitted by vehicles in a downward direction or at or near ground-level; characterised in that the system further comprises: a distributor operatively coupled to the first collector, the second collector, and to the component detection system, wherein the distributor is configured to couple the first collector to the component detection system when a passing vehicle emits exhaust emissions in an upward direction or at an elevated position, or couple the second collector to the component detection system when a passing vehicle emits exhaust emissions in a downward direction or at or near ground-level; and wherein the component detection system is configured to receive a sample of the exhaust emissions of the passing vehicle directed by the distributor, and to determine the concentration of one or more constituents present in the received sample of exhaust emissions. a component detection system configured to determine the concentration of one or more constituents present in a sample of vehicle exhaust emissions; a first collector positioned above the surface of the roadway, the first collector having one or more openings configured to receive exhaust emissions emitted by vehicles in an upward direction or at an elevated position; a second collector positioned on or near the surface of the roadway, the second collector having one or more openings configured to receive exhaust emissions emitted by vehicles in a downward direction or at or near ground-level; characterised in that the system further comprises: a distributor operatively coupled to the first collector, the second collector, and to the component detection system, wherein the distributor is configured to couple the first collector to the component detection system when a passing vehicle emits exhaust emissions in an upward direction or at an elevated position, or couple the second collector to the component detection system when a passing vehicle emits exhaust emissions in a downward direction or at or near ground-level; and wherein the component detection system is configured to receive a sample of the exhaust emissions of the passing vehicle directed by the distributor, and to determine the concentration of one or more constituents present in the received sample of exhaust emissions. a distributor operatively coupled to the first collector, the second collector, and to the component detection system, wherein the distributor is configured to couple the first collector to the component detection system when a passing vehicle emits exhaust emissions in an upward direction or at an elevated position, or couple the second collector to the component detection system when a passing vehicle emits exhaust emissions in a downward direction or at or near ground-level; and wherein the component detection system is configured to receive a sample of the exhaust emissions of the passing vehicle directed by the distributor, and to determine the concentration of one or more constituents present in the received sample of exhaust emissions.