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.
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.
- 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.
- More expensive than a comparable three-way junction like the trumpet.
- Difficult to build in structural terms.
- Difficult to expand or increase capacity.
The Directional T comes in three flavours. The original - and the one that really is a Directional T - doesn't exist in this country. In the US it's considered the original and traditional form, and one was planned here in Glasgow. There's space and stub sliproads for it between junctions 15 and 16 of the M8, but it was never completed.
A bit of extra bridge construction allows one route to take priority, with the other being subordinate and connecting by left-hand sliproads. This turns it into a Semi-directional T
One more bridge allows the two right-turn sliproads to cross again, giving priority to one of the turning movements. This is a Triangle interchange, from its very symmetrical shape.
You can twist any of 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.
With thanks to David, Dave Robinson and Peter Edwardson for information on this page.