First, if you want to drive efficiently when approaching a junction, avoid using the auxiliary brakes. They transfer the truck’s kinetic energy into heat and that results in lost momentum. When approaching a junction, the less braking you do, the better it is for fuel economy. It doesn’t matter if you use the service, auxiliary or engine brakes, it’s still a waste of energy.
To find a good balance, it’s usually better to coast when the truck has the most kinetic potential. As a driver trainer, I recommend, whenever traffic allows, to slow down from cruising speeds to about 50 km/h by coasting. This way, you are using the air friction to slow you down, rather than the brakes. This is certainly achievable on flat approaches and slight inclines. Downhill, it might be necessary to use the auxiliary brakes to help maintain a sensible approach speed.
When driving on flat conditions at speeds of under 50 km/h, use the automatic auxiliary brakes. The auxiliary brakes alone are not very effective at reducing speed when shifting gears and using them on their own can feel uncontrolled and uncomfortable. A truck driving at less than 50 km/h has very little kinetic energy left, so there’s no harm in using the service brakes a touch during the approach. It certainly won’t cause significant wear. Using the automatic auxiliary brakes and braking by foot also gives you the chance to anticipate what others are doing and maintain a comfortable approach to a junction. This is much more important when it comes to potential cost savings than trying to prolong the life of service brakes by using the auxiliaries at slow speeds.
When coasting to approach junctions on flat roads, I know for a fact that a 37-tonne combination will coast for 2 kilometres. So this is by far the cheapest way to approach a junction, as long as you start anticipating at a good distance. On a flat stretch of motorway before exiting this shouldn’t be an issue, as the junction signs are usually visible very early.
I can accept that a two-kilometre approach is not always realistic, but the cost savings make it well worth at least trying to identify a good balance between not braking and having a sensible approach distance.
driver development at Volvo Trucks