The challenge of linking two places across an inhospitable gap is what makes bridges one of the most exciting types of all engineering structures. It seems a shame, then, that so many travellers hurry across interesting bridges without stopping to look at them. For children, curriculum links to science, technology, history and geography are obvious.
The Bristol area has the Clifton Suspension Bridge and the two Severn road bridges. The second, the M4 Severn crossing - the UK's longest bridge, opened in 1996 - is a cable-stayed bridge. The first crossing, along the M48, is a suspension bridge. When it first opened, in 1966, it was the seventh longest suspension bridge in the world and pioneered an aerodynamically shaped deck. Seeing them side by side makes the two design principles clear.
I visited the second Severn bridge in what was described to me as "ideal" weather: it was blowing a gale and stinging spray was coming off the water. As I leaned into the wind I knew that it was true. Big bridges are built for that sort of day.
Peter Head, the government engineer on the crossing, said: "The construction of the bridge was one of the great engineering challenges of this century. There's a tidal range of 12 to 15 metres, which means that half the time we were building in the dry and half the time in 12 metres of water." Various props were needed to carry out the job: "all sorts of wierd and wonderful craft, great platforms with legs and cranes to sit on them," he recalled.
The winds blow hard in the Severn estuary, but the second crossing has never yet had to be closed for high winds, thanks to some innovative windshielding. All of this, and much more, is explained through photographs, diagrams and models in the nearby Severn Bridges visitors' centre.
Adam Matthews, key stage 2 co-ordinator at Walwayne Court primary school in Trowbridge, Wiltshire, was extracting the maximum value from the school trip to the bridges and was planning to go on to the Clifton Bridge. "We've been focusing on structures and forces," he said. "We've looked at different kinds of bridges and used art straws to build models, looking at things like balance and testing them to destruction." (The destruction element was particularly popular.) Isambard Kingdom Brunel's Clifton Bridge, a very early suspension bridge (opened in 1864), is on a much smaller scale than the Severn bridges. This has advantages - you can walk on it and see the structure more clearly, for one thing. The visitors' centre is excellent and includes a model suspension bridge which children can walk on and feel the movement as the structure takes their weight.
Mr Matthews pointed out that together the three Bristol-area bridges bring to life much of what he teaches about forces and structures. "Children can see the balance between gravity and upthrut," he said, "and the putting into practice of ideas about materials, and lightness and shape."
Late this spring, Explore@Bristol, an extensive interactive hands-on science centre, will open in the harbour area of Bristol. It will feature a special section on bridges.
Almost any journey of reasonable length will take you within reach of a bridge worth visiting.
North from Edinburgh, for example, you can see the contrasting Forth bridges - the massive cantilever railway bridge of 1890, and the mile-and-a-half suspension road bridge. The Forth road bridge has no visitors' centre, but can accommodate school visits, offering talks, slides, a visit to the control room and a viewing platform. Tel: 0131 319 1699.
In Dundee, you can pick up the story of the Tay Bridge which collapsed in 1879, taking a train and its 75 passengers with it. This was a Titanic-style tragedy in its time and was responsible for raising the quality of engineering afterwards. The stumps of the original bridge are visible and there is information and mementoes of the tragedy in the McManus Galleries, Dundee. Tel: 01382 432084.
The Humber Bridge, built in 1981, is huge - 1,410 metres between 155 metre towers. Until 1998 it had the longest suspended span in the world; now that record is held in Japan. Views of the estuary are spectacular. There is a viewing area and a small visitors' centre. Tel: 01482 640852.
The first iron bridge in the world - an elegant cast iron arch built in 1779 with a 100ft span supporting a roadway - is at Ironbridge, near Telford, Shropshire. It was designed by Thomas Pritchard for builder Abraham Darby and is part of the Ironbridge Gorge group of museums. Tel: 01952 432166.
The half-mile medieval bridge at Swarkestone, just south of Derby on the A514, is a narrow packhorse bridge across the Trent and the surrounding flood plain. It has several claims to fame: it is the longest stone bridge in the country, it marks the furthest southerly point reached by Bonnie Prince Charlie's forces in the 1745 Jacobite rebellion, and in the English Civil War a pitched battle was fought on it. Details from Derby Heritage Centre, tel: 01332 299321. Local historian Richard Felix will give on-site talks.
Tower Bridge, one of London's most famous landmarks, is a bascule bridge and is raised for river traffic. The machinery used to be driven by steam and seeing it is part of the Tower Bridge Experience, which welcomes school parties. Tel: 020 7378 1928.
* Severn Bridges visitors' centre, Severn Beach, Bristol BS35 4HW. Tel: 01454 633511.
Open April-October daily except Mondays, November-March weekends. Adults pound;1.25, pound;1, children 50p.
Clifton Suspension Bridge visitors' centre, Bridge, House, Sion Place, Clifton, Bristol BS8 4AP. Tel: 0117 974 4664.
Open daily. Groups pound;1.30, children 90p. Teachers' notes free.
Explore@Bristol tel: 0117 909 2000. www.at-bristol.org.uk