Directional T
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If you're looking at terminating one motorway standard road on another, and you have the money to do something special, the Directional T is where it's at. It uses up more land than a trumpet and involves much more engineering work too. But with plenty of advantages over its smaller cousins, and three flavours to choose from, the Directional T is worth the investment.
Advantages
- Handles high volumes of traffic with ease.
- Easy for the motorist to use.
- Three designs allow adjustment of the design to suit almost any situation.
Disadvantages
- More expensive than a comparable three-way junction like the trumpet.
- Difficult to build in structural terms.
- Difficult to expand or increase capacity.
Variations
The Directional T comes in three flavours, as we know. The most common (pictured above) gives equal priority to each direction to and from the terminating road. It gives the other road overall priority.
An additional bridge span allows two of the sliproads to cross each other again, giving priority to one turning movement and keeping the other road's overall priority.
Third, it's possible to build the junction so that all three arms share equal priority. This is regarded as the traditional version in the US, but none were ever built in this country. Space was made for one, but it was never built, between junctions 16 and 17 of the M8 in Glasgow.
You can twist these interchanges around to fit whichever way you like - such as here on the Runcorn Expressways.
Lastly, it's possible to make this a restricted access four-way junction, allowing what would be the terminating road to continue straight through. This can be seen at the M1/M25 interchange.
Where to Spot Them
Scotland is fond of the design pictured at the top of the page, with them cropping up at the M8/M9 and M73/M74 junctions, and a slightly embellished one for the M90 spur near Perth. In England, we prefer to give one direction priority, as at M27/M275 and both ends of the M18.
With thanks to Dave Robinson and Peter Edwardson for information on this page.
