The roadway, which spans the estuary in one elegant 2.2km arc, is revealed as a construction of more than 100 prefabricated steel boxes. The view through the small access doors from one section to the next, and on, is breathtaking: like the last thing glimpsed by the victim of some colossal serpent. There's constant noise, a low metallic moaning, as the steel of the bridge responds to the pressures of wind and traffic. Every so often there's a thunderous roar as a juggernaut passes overhead. Roger Evans's eyes light up: "That's another pound;16.70 for us."
Why are we here, inside one of the civil engineering triumphs of the late 20th century? The Humber Bridge is a powerful symbol of regeneration for a region which has suffered more than its fair share of industrial decline.
It is also, as Roger Evans says, "100 per cent physics" - and, as such, an ideal subject for the "physics visit", a key element of the Salters Horners advanced course these students are taking.
As the tour progresses, the students see and hear the evidence that will help them answer such questions as: why is the deck shaped like an inverted aeroplane wing? How much lower is the bridge in summer, when the cables expand and allow the roadway to sink towards the water? How come the tops of the two 155m towers are further apart than the bases, even though each is vertical?
Standing inside the northern cable anchorage point and staring down at the splayed-out bundles of wires at the point where they plunge into a 190,000-tonne block of concrete, the students get a sense of the scale of forces at work here. Then it's back indoors for a debriefing. One student wants to know about the dumbbell-like objects attached to the secondary cables. These, it turns out, are dampers, added to combat vibrations set up by certain combinations of wind and rain.
The answer feeds straight back into the physics the group has just been studying - vibration and resonance - and leads to a lucid explanation of why the Millennium Bridge wobbled and why the Humber Bridge does not.
Back at college, students were positive about the trip. For Mary Wall, "the most interesting part of the afternoon was standing inside a box section, knowing that there were only a few millimetres of steel around you."
Anthony Harrison found the visit brought together different elements of his work: "It was useful to see the principles of waves and other physics and maths lessons being related to the structure of the bridge. The visit showed how physics is relevant to the real world and not just experiments in college. It's fascinating to see how, with the right construction methods, dozens of HGVs can be held up on 9mm sheet metal."
Zhou Liang, from China, found "many things which relate to what I am learning in college, like the effects of the cables getting longer as the bridge heats up, meaning that the height of the bridge depends on temperature. To get a look inside the bridge and see the clever features of its construction was fantastic."
Howard Darwin, head of physics at John Leggott, explains that the college was one of the pilot centres for the Salters Horners course in 1998 - "In fact, we wrote part of the syllabus" - and that the "physics visit" was rapidly established as an integral part of the course. Students can either go on one of a number of arranged visits or devise their own. The aim is to help them experience the excitement of physics, stimulate interest and drive home "the relevance of physics to work and life". It's also intended to develop communication skills: "We get them to write a report - not a scientific paper, more like a news-paper article to explain the physics to the public."
John Leggott College has an international reputation for attracting more students into science subjects, and producing better results, year after year, against all the trends. "We currently have around 70 A-level physics students (A2 year), from just under 50 in 1999 at the outset of Salters Horners physics. AS physics numbers are more difficult to compare as we are only a couple of years into the new system where many more do AS. But, for each of the past two years, we have had more than 130 enrol to create seven full AS classes." Which, he adds, is right at the limit of the college's current capacity. It's a huge intake, especially when, nationally, applications for A-level physics have been declining steadily for more than 20 years.
The college is the biggest centre in the UK for Salters Horners physics, and attracts large numbers of overseas students - especially from China, Russia and the Middle East. The visits had been a factor in this, Howard Darwin says: "Not a huge factor, but one of many." This year, 40 students went to the National Museum of Film, Photography and Television at Bradford, 30 to the Drax power station, 15 to a nearby laser engraving company, and 12 to the Leeds general infirmary to investigate the physics of ultrasound testing. "A big hit in previous years was the Nestle chocolate factory, where students were amazed to see how much physics, and how many millions of pounds, went into getting just the right crunchiness into a Kit Kat bar."
Howard feels it should not be beyond any college to find enough local enterprises to offer suitable science-related visits: "Once you start to look around, you find lots of organisations are willing to help." Many enterprises recognise the value of the visits in helping tip students in their direction. One former student chose to study civil engineering at Cambridge as a direct result of her visit to the Humber Bridge two years ago.
Derek Denby, director of the John Leggott's Science Centre of Excellence, says its expertise "stems from a vision to do something to halt the decline in sciences that we've seen nationwide. We needed to do something. The physics visits are an illustration of one of the things we could do.
"In 1997-98, funded by the local skills councils, we set up the Science Centre of Excellence. It was an umbrella for a variety of different initiatives." With the funding came the need to disseminate the ideas, and Derek Denby finds himself in other colleges two or three times a week, explaining how to turn students on to science. "You have to start early," he says. "We start with 11-year-olds who come to our science summer schools." He accepts that it can be difficult for colleges with declining science numbers to turn this around, but hopes that his centre's work is now "setting up ripples across the country".
John Leggott College, West Common Lane,Scunthorpe DN17 1DS Tel: 01724 282998 www.leggott.ac.uk
The Humber Bridge, Ferriby Road, Hessle, East Yorks HU13 OJG Tel: 01482 647161 www.humberbridge.co.uk
Salters Horners Advanced Physics, Science Curriculum Centre, University of York, York YO10 5DD www.york.ac.ukorgsegsaltersphysics