gistfile1.txt

$array = [
    '0' => [
        'data' => [
            'traffic_in' => [
                '1',
                '2',
                '3',
                '4'
            ],
            'traffic_out' => [
                '1',
                '2',
                '3',
                '4'
            ],
        ],
    ],
    '1' => [
        'data' => [
            'traffic_in' => [
                '1',
                '2',
                '3',
                '4'
            ],
            'traffic_out' => [
                '1',
                '2',
                '3',
                '4'
            ],
        ],
    ]
];

foreach($array as $set) {
    foreach($set['data'] as $dataSource => $values) {
        $dsArray[$dataSource][] = $values;
    }
}


$summed = [];
foreach($dsArray as $dataSourceName => $sets) { 
    if(!isset($summed[$dataSourceName])) {
        $summed[$dataSourceName] = [];
    }
    foreach($sets as $set) {
        for($i=0; $i<count($set); $i++) {
            @$summed[$dataSourceName][$i] += $set[$i];
        }
    }
}

print_r($summed);

that's really ugly but does the trick. Need a more elegant way at some point