Perhaps I misunderstood my instructors when I did my electronics apprenticeship back in the 60s, but the "induced electric field" you speak of is an "electro-magnetic field" so by introducing a magnetic disturbance into this field, this alters the field, which is the signal the loop is designed to detect. I can't see how dragging non-magnetic aluminium through the induced field will be any more effective than taking a "magnet" through it.
(I have some background in this stuff).
The key thing is it doesn't have to be magnetic material. A magnet as such will have virtually no effect, it's got more to do with the material the magnet is made of than the fact it is magnetic.
When a field passes through a loop it acts like a short circuited transformer. The shorted current that flows in the shorted turn acts against the field that is emitted from road sensor and it is possible to detect this.
There' two ways induction loops can detect metal. The first is based changes in the field when something magnetic like iron is put in the field. In this case the best mode of detection is in the centre. The other is eddy currents being induced in the conductive material - like a shorted turn on a transformer. As the article states this is more often the mode of detection. In this mode you want the field from the sensor to pass through as much crossectional area as possible. A vertically bike has a very low crossection when looking up from the road. However a car has quite a large crosssection, being wider then it is high.
The field from the road loops shoots vertically upward at the centre. So if you are at the center of the loop the best way for detection is to tilt the bike over so for vertical field can past through the some of the "loops" on the bike - if the bike is vertical there is not loop. In effect you are making the area of the bike larger as seen from the ground. (A steel bike may be detected in the centre through the first mode of detection.)
At the sides of the road loop the field bends outwards in all directions (just like the pattern of iron filings at the end of a magnet). This means that at the edges of road loop the bent field can actually pass through the side of the bike. Looking from the side the bike has a wider cross-section and therefore it can be detected. If you want to improve detection at the sides you can lean the bike over towards the centre of the road loop.
The side by side figure 8-pattern road loops are more complicated to explain. Just accept they are design to detect at the centre and should pickup a vertical bicycle.
This site shows the spots:
http://www.geocities.com/siliconvalley/ ... ensors.htm
(As a side point the square loop can detect the bike at the leading and trailing edges as opposed to the sides as shown, however, you would have to rotate the bike so the large area of the biek was along the side - which not practical.)