.passenger{
    width: 40px;
    height: 40px;
    color: black;
    position: absolute;

    transition: left 1s ease-in-out;
}

.passenger::before{
    content: attr(data-count); /* ← ここがポイント */
    width: 35px;
    height: 35px;
    background-color: paleturquoise;
    border-radius: 50%;

    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%); /* ← 中央配置 */
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 12px;
    color: black;
}