Braking distance, friction and behaviour – Findings, analyses and recommendations based on braking trials

The recommended braking distances in the Danish Road Standards and Guidelines are in the main based on earlier American findings. In order to be able to assess the validity of the recommended braking distances in relation to contemporary vehicles and motorists in Denmark, the Danish Road Directorate has conducted a study designed to shed light on braking behaviour and braking distances among ordinary (non professional) motorists, at different speeds.

This was done through a measurement programme in which 22 test drivers performed braking manoeuvres at different speeds (80, 110 and 130 km/h). The majority of the test drivers who participated were recruited from among non professional drivers. However, 6 out of the 22 test drivers were professional test drivers with extensive experience in advanced driving technique. Two different recent cars with ABS brakes were used as test cars. The braking manoeuvres were carried out on dry and wet road on 3 test tracks with different friction. The majority of the manoeuvres performed were emergency stops, in which the test driver was required to bring the vehicle to a complete standstill as quickly as possible. In addition, a small number of comfort braking manoeuvres were performed in which the test driver was required to bring the vehicle to a comfortable stop.

The main findings of the measurement programme (a total of 172 emergency stops and 23 comfort braking manoeuvres) are presented below in bullet form:

·   The non professional test drivers generally have a longer braking distance compared with the professional test drivers. The difference is very individual however. The majority of the non professional test drivers have an average braking distance that is 0-20% longer than that of the professional test drivers. A few non professional test drivers, however, have a significantly longer braking distance.

·   The greatest difference between the professional and non professional test drivers is seen on wet road surface.

·   Generally, the largest spread in braking distance among the non professional test drivers is seen at high speed and in situations on wet road surface.

·   The more times the non professional test drivers performed an emergency stop, the better they became at braking. The difference between the professional and non professional test driver was therefore reduced after 6-7 braking trials.

·   The test drivers’ pressure on the brake pedal during braking indicates that the professional test drivers apply far more rapid and generally firmer pressure on the brake pedal than the non professional test drivers.

·   The analyses also indicate that the vehicle’s deceleration is independent of brake pedal pressure as long as it simply exceeds 15-20 kg. Where there is less brake pedal pressure, deceleration is reduced significantly.

·   The average deceleration values among the professional test drivers are between 8.1 and 8.6 m/s² on dry road and between 7.2 and 8.5 m/s² on wet road.

·   There is apparently only a small difference between the two test cars used.

·   The significance of friction for the braking distance on dry road is negligible. On wet road however, the braking distance increases where friction is lower.

·   A good correlation was found between braking distance on wet road, friction and initial speed. This correlation is expressed in a formula obtained from regression analyses.

·   The average deceleration in comfort braking manoeuvres was found to be 3.2 m/s²

The completed measurement programme is based on 2 fairly new test cars with new summer tyres. In order to determine the significance of the choice of vehicle and tyre for overall braking distance, the findings of other relevant studies were reviewed. Depending on make of tyre, winter/summer tyres, tread depth, loaded/non-loaded and make of car, great individual differences affect a vehicle’s overall braking capability. The condition of the tyres (including tread depth), especially, is highly significant for braking distance on wet road. The finding is that a vehicle with poor braking capabilities will result in a braking distance some 30-60% longer than the test cars used in the measurement programme.

Based on the findings of the measurement programme and the knowledge obtained concerning the significance of other parameters for braking distance (choice of tyre, vehicle, etc.), we have sought to establish a new set of recommended braking distances. These are based on “worst case scenarios”, i.e.

·   wet road surface with poor friction (friction=0.4 – minimum requirement for roads in operation)

·   driver with tentative braking behaviour

·   vehicle with poor braking capability

The new recommended braking distances are thus based on calculated braking distances with friction of 0.4 for professional test drivers in the test cars used. In addition, we added a behavioural increment of 30% to reflect the tentative driver, and to this we added a vehicle increment of 45% to reflect a vehicle with poor braking capability. The new recommended braking distances for speeds of 80, 110 and 130 km/h are shown in Table 7.1. The braking distances are assessed as somewhat ”cautiously determined” with a good safety margin in relation to the cars and motorists driving on Danish roads.

For speeds below 90 km/h, the new recommended braking distances are almost identical with the existing ones in the Danish Road Standards and Guidelines. At greater speeds, e.g. 130 km/h, the new recommended braking distances are approx. 25% shorter than the existing ones (see Figure 6.2 page 63).