Descending I-70 in a Spring Squall on All-Season Tires
You are eastbound on I-70 descending from the Eisenhower Tunnel at 5:30 p.m. in late April. The posted limit is 65 mph, but a spring squall has dropped the temperature to 28°F and slush is building in the wheel tracks. Your SUV has all-season tires with about 6/32 inch of tread, and you are not towing. A variable message sign you passed said 'TRACTION LAW IN EFFECT.' Traffic is moving at about 40 mph. The grade ahead is 7% for roughly six miles. Your brakes feel normal, and you are in the right lane. A line of cars is close behind you. Your exit at Georgetown is two miles ahead, and you can smell the brakes from a truck ahead.
1. What does the traction law require, and how does your tire condition factor into the decision to continue?
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The traction law is in effect, so the SUV must have tires rated for mud and snow or all-weather with at least 3/16 inch of tread, or use chains/alternative traction devices. Your 6/32 inch is exactly 3/16 inch, so it meets the bare minimum on paper, but there is little margin for a 7% descent in slush. All-season tires are not winter tires, and tread wear reduces snow grip. Check that the tires carry the M+S marking; if they do not, or if tread is below the minimum, do not continue under the law. The message sign is not a suggestion-it is the condition of the road.
2. How should you control speed and braking on a 7% grade in these conditions?
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On a 7% grade, use a lower gear and let engine braking do most of the work. Do not ride the brakes continuously; that builds heat and can cause fade. Brake firmly enough to slow, then release and let the engine hold the speed. Keep a large gap ahead, avoid cruise control, and stay well below 65-40 mph traffic is already telling you the safe pace. If you smell hot brakes from the truck ahead, do not follow closely; that driver may be in trouble. Your exit at Georgetown is a chance to pull off and let the brakes cool if they feel soft.
3. What should you do if the rear starts to slide or the brake pedal begins to feel soft?
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If the rear starts to slide, ease off the accelerator and steer smoothly in the direction you want the front of the vehicle to go; do not jerk the wheel or slam the brakes. If the brakes fade, downshift immediately, use engine braking, and look for a safe escape-an exit, a wide shoulder, or a runaway truck ramp-rather than riding a soft pedal into the next curve. Turn on your hazard flashers to warn traffic behind you. Once stopped, set the parking brake, let the brakes cool, and do not continue until you have a safe plan.