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Entryways must be provided along the me- dian berm to allow the driver to align the vehicle with the berm. A road grader 95 can shape the berm base to a width nar- rower than the vehicle's track. ESCAPE LANES The best alternative for effective vehicle restraint involves the use of es- cape lanes. They ideally perform their function without causing the vehicle to roll over. Model testing of an 85-ton vehicle was performed on an escape lane constructed of dry, fine-grained homogen- eous sand and a lane constructed of com- pacted fire clay. The arrangement of the model test consisted of (1) accelerating a scaled model vehicle down a curved ramp to obtain various entry velocities, (2) allowing the moving model to enter the escape lane test bed, and (3) measur- ing its stopping distance at the various entry velocities and escape lane slopes. Figure 5 shows a comparison of these tests on a sand test bed with theoretical analyses. Some escape lane recommenda- tions developed are-- 1. Entry to the lane should be marked by appropriate signs. Regardless of the material used, it must be free-draining so that freezing will be delayed during cold weather and it will not readily compact. It must also be a material that can be readily smoothed out after use and that can be maintained with a minimum of effort. 7. The length of the escape lane should be based on the largest size vehi- cle traveling at a realistic maximum speed. The length of the lane will also be a function of the terrain; a lane lo- cated with a downslope would be longer than a lane located with an upgrade. 8. A barrier constructed from sand, gravel, or any available overburden mate- rial should be positioned at the end of the escape lane to prevent a vehicle from traveling over an embankment, in the event the vehicle is not stopped in the escape lane area. 2. Entry to the lane should be a smooth transition from the haulage road, thereby minimizing the steering require- ments of the operator. 3. Width of the lane should be reason- ably greater than the width of the largest haulage vehicle. 4. Depth of loose gravel or sand mate- rial should taper from a minimum at the entryway to two times the maximum ground clearance of any vehicle expected to uti- lize the escape lane. 5. The depth of the arresting material should be graduated along the initial en- tryway of the ramp so that a vehicle will not be stopped too abruptly. 6. Material used in the escape lane should be of an unconsolidating nature; pea gravel is the most common material used in public highway construction. 9. The arrester bed material should also ensure that once a vehicle is stopped it will not roll back. Mainte- nance is necessary to keep the ramp in the proper condition. The ruts rmist be smoothed out and the surfacing material loosened frequently. As the material be- comes infiltrated with dirt and other fine materials, it must be removed and replaced with clean material. 10. The optimum escape lane would con- sist of an uphill grade; however, in ac- tual practice, the required location may be along a downgrade having the necessary length for stopping the vehicle. 11. Model tests showed that an escape lane with a median berm reduced the stop- ping distance approxlmately 50 pct for a fully loaded 85-ton vehicle traveling at 40 mph. But since the median berm would be hard to maintain, it should be used where the escape lane length is limited.