Outside-in or Inside-out

It’s a problem designers have been wrestling with since the divided motorway was first devised — when designing the first stage of a highway do you build it so it can be developed from the outside-inwards with extra travel lanes, or from the inside-outwards? The European tradition from Hitler’s autobahns onward has been to build 2x2-lanes tightly around the median divider, so that the whole roadway, breakdown lanes, medians and all is no more than about 22m (72’) across. And widening has to be outwards. That certainly minimizes initial cost and impact. In some places it may be all that can be achieved politically.

With the need to add capacity you have to add travel lanes on the outside. In one respect this is easier. Construction equipment, materials and workers don’t have to cross the mainline to get to the worksite. It is often very bitsy and awkward however. Much of the work is on embankments and over ramps, split between both sides. More landscaping will be disturbed, and neighbors. In addition ramps themselves will usually have to be relocated and rebuilt in the inside-out widening.

The North American approach at first was similar to the Europeans as seen in various NY/NJ and Washington DC parkways, the Aroyo Seco LA’s first mwy, and the Pennnsylvania Turnpike — all constructed tightly around the median. But during the big boom in building US interstates, 1958 to 1968, it became the general rule that you must provide a wide grass median of 15m to 20m.

Seppo Silla a veteran of geometric design issues at the Fed Hwy Admin says that a mixture of aesthetic and safety issues led to wide grass medians becoming virtually the norm from the late 1950s on. The median would reduce the aesthetic impact of pavement on the landscape, it was said, masking the scale of the roadway as a whole, allowing the separate roadways to follow their own separate paths and elevations. At first a standards was 50’ (15m.) Soon safety studies showed the benefit of a 30’ (9m) clear zone for errant vehicles to recover without hitting a deadly obstacle suggesting the need for a minimum 64’ (20m) to allow for central overpass bridge piers 4’ wide. later an argument was advanced to increase this to 80’ (24m). AASHTO design standards directed most high standard US roads to be built with 20m to 24m medians from the 1960s. The Canadian approach was similar.

Inwards widening of travel lanes started as a second-best procedure where adjacent right-of-way was unavailable, too expensive or politically difficult to obtain. However as the central concrete or Jersey barrier has been refined, it plus a 2 to 3m inside shoulder has been generally accepted as being equally safe as a wide grassed clear zone, so it is generally possible to get at least one extra travel lane each direction and paved shoulders building inwards, on the grass median. And of course anti-highway groups have a much ahrder time fighting an inwards-widening. Outside widening naturally stirs up neighbors, who may have to sacrifice land or see traffic getting loser.

Pressures to make provision for HOV lanes or transit have reinforced the practice of reserving a wide median in new highways in North America. SJ Hills pike, CA-125, Dulles Toll Road and Gway, Toronto’s 407-ETR, and many new Texas roads are designed for outside-in development. Christian Geroneau in “Transport in Europe” (Artech House, p78) lauds the practice and laments the shortsightedness of narrow European mwys . (Seppo Silla FHWA 202 366 0312)