Ramp grades

The effect of vehicle speed on crash likelihood and injury severity has a strong theretical basis in physics and biomechanics, and has been demonstrated many times through empirical study. Moreover, the effect of speed humps and cushions as a means of controlling speeds is well established. Combining vertical deflection with cycling and walking crossings of roadways is well established practice (Fig. 1 and Fig. 2), both at mid-block path crossings and raised thresholds at T-junctions. Typical practice, in accordance with Austroads, is to nominate ramp grades of 1:12. However, in practice the as-constructed ramp grades can vary markedly (often being shallower) due to the road camber. The question then arises as to what the potential effect on speed will be if the grade is compromised. Further, I would argue that 1:12 is too shallow for many situations - for example, local residential streets and at T-junctions.

Path priority crossing (St Georges Road, Northcote)

Figure 1: Path priority crossing (St Georges Road, Northcote)

Raised threshold (Glenferrie Road, Malvern)

Figure 2: Raised threshold (Glenferrie Road, Malvern)

VicRoads has published a draft guidance note on raised platforms. While specifically concerned with raised platforms at signalised intersections the formulae in this note provide an indication of the relationship between ramp grade, vertical acceleration and therefore motorist speed selection. While the empirical data supporting these relationships is somewhat unclear, and likely based on empirical studies undertaken decades ago, it provides some insight into the impact of grades.

As a quick means of assessing the impact of grades the relationship between vehicle speed and vertical acceleration is shown below. An interactive version is available here.

Figure 3: Relationship between ramp grade, vehicle speed and peak vertical acceleration

Taken at face value the implications for a 1:12 ramp are as follows:

  • At vehicle speeds at or below 21 km/h the motorist will experience vertical accelerations below 0.5 g, which will not result in the motorist adjusting their speed

  • At speeds between about 21 and 25 km/h the motorist may slow somewhat

  • At speeds above 25 km/h potential vehicle damage may result (e.g. scraping, suspension “bottoming” out)

  • At speeds above 30 km/h the vehicle occupants will be exposed to vertical accelerations of 1 g or greater, which would be considered “uncomfortable”.

What would be interesting would to verify these g-thresholds do indeed result in the predicted motorist speeds, and what is the effect of ramp grade on extreme speed (i.e. the tail of the speed distribution). That is, the few motorists who travel at wildly excessive speed that present a disproportion risk of injury.

Finally, Dutch practice is to use 1:4 ramps at raised thresholds at T-junctions. According to the above model this would mean a motorist would experience 1 g at 10 km/h. To my view these sort of speeds are perfectly reasonable to expect of motorists entering and leaving minor streets. I’m thinking of locations like this, this and this. These lower speeds provide greater opportunity for motorists to look and see pedestrians on the footpath, as well as motorists and bicycle riders on the main road. And the likelihood of a collision resulting in serious injuries at these speeds are very low.1

  1. To be fair, they’re not zero though. There have been instances of trucks turning into side streets striking and serious injuring pedestrians waiting on the footpath. But this isn’t a speed issue - it’s about side underrun protection on the trucks (and trailers) and physical measures that prevent vehicles from encroaching onto footpaths and kerb outstands.

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