Types of validity and threats to validity ......................................................17
4.2 Threats to validity in studies evaluating the relationship between speed and
road safety..........................................................................................................19
4.2.1 Errors and bias in speed data........................................................................... 19
4.2.2 Incomplete accident reporting and changes in the level of accident reporting 20
4.2.3 Regression-to-the-mean................................................................................... 20
4.2.4 Long term trends in accident rates................................................................... 22
4.2.5 Major changes in traffic volume ..................................................................... 22
4.2.6 Inadequate control of risk factors influencing accident occurrence................ 23
4.3 Control for threats to validity in various study designs...............................26
4.3.1 Commonly employed study designs................................................................ 26
4.3.2 Ways of introducing speed-influencing measures........................................... 27